1.The assistant _____ (call for) me at 9 in the morning.

其实前面的两位朋友都已说得非常清楚.这是现在分词、过去分词作定语的问题：
1.The assistant serving her = The assistant who sverved her ——服务她的那个店员
2.The assistant served （by） her = The assistant who was served by her
被她服务的那个店员
很明显,用 served 的话,后面还要加 by,否则句子是不通的.—— 如果真要表达 2.的意思.
原句要表达 1.的意思,所以要用 serving,类似的例子：
The boy standing over there is Tom's brother .站在那边的男孩是汤姆的兄弟.
The book written by a student has been published.由一个学生写的那本书已经出版了.

应为 The assistant who serves her 或serving her did not
serving的意思是那个正在服务（协助）她的助理
who serving则不为完整句
如有问题,欢迎继续追问：）

serving是做定语的 .对名词的一种解释 ,served 后面一般跟by sb.做被动
下面是对后置定语的解释：
后置定语
形容词是表示人或事物的性质、特征或属性的一类词.它在句中可以充当定语,对名词起修饰、描绘作用,还可以充当表语、宾语补足语等.形容词作定语修饰名词时,一般放在被修饰的名词之前,称作前置定语.但有时也可放在被修饰的名词之后,称作后置定语. 1. 当形容词修饰由every-、some-、any-、no-加body、one、thing等构成的复合不定代词时须后置.例如： Have you ever met anyone famous ? 你曾经见过名人吗? He did everything possible to help us . 他做了一切可能做的来帮助我们. There is something wrong with the computer . 这台电脑出毛病了. 注意：若something 前面有定冠词修饰,形容词不用后置.例如：There , on the table , was the mysterious something that they had been looking for . 看,在桌子上,正是他们一直孜孜以求的神秘的东西. 2. 形容词组作定语须后置,其功能相当于一个定语从句.例如： _________ to take this adventure course will certainly learn a lot of useful skills . A. Brave enough students B. Enough brave students C. Students brave enough D. Students enough brave They are the boys easiest to teach . 他们是最容易教的孩子.（划线部分相当于who are easiest to teach） The basket full of some fruits belongs to the old woman . 盛满各种水果的这个篮子是这位老妇人的.（划线部分相当于which is full of some fruits ） Those brave enough to take the course must be the top students in the grade . 那些敢于选修这门课程的学生一定是本年级的尖子生.（划线部分相当于who are brave enough） 3. 两个形容词用and 或or连接作定语,进一步说明被修饰名词的作用,此时须后置.例如： Power stations , large and small , have been set up all over the country . 大大小小的水电站已经在全国各地兴建起来了.（划线部分相当于which are large and small） Every book , new or old , should be put in the room . 不论新书还是旧书都应该放在这个房间里.（划线部分相当于which are new or old） 4. 形容词else常位于不定代词或疑问代词后作定语.例如： Anything else I can do for you ? 我能为你做些别的事情吗? What else did they say ? 他们还说了些什么? 5. 形容词比较等级结构修饰名词时须后置.例如： The Smiths need a car garage twice larger than this one . 史密斯夫妇需要一个比这个大一倍的车库. He has booked the seat farthest from the door on the train for us . 他已经为我们预定了火车上离车门最远的座位. 6. “基数词+表示时间或度量单位的名词+形容词”结构可用来修饰名词,起定语作用.此时,该类结构的词组须后置.例如； Yesterday , a Mr Brown , 28 years old , came to visit you . 昨天,一位二十八岁,自称是布朗先生的人来拜访过你. There used to be a tall building about 50 metres high . 过去,这里曾经有一座大约50米高的大楼. 7. 部分以a-开头的表语形容词,如：alive , alike , alone , awake 等,有时也可以作定语修饰名词.此时,须将该形容词置于被修饰的名词之后.例如： She was the only person awake at that night . 她是那天晚上惟一一个醒着的人. You are the happiest children alive . 你们是当代最幸福的孩子. 8. 形容词responsible 作“可信赖的；可靠的”解时,作前置定语；作“有责任的；应负责的”解时,须作后置定语.例如： He is a responsible man . 他是一个可以信赖的人. The man responsible should be their manager . 负责任的应该是他们的经理. 9. 形容词present 作“现在的；目前的；现存的”解时,作前置定语；作“出席的；在场的”解时,须作后置定语.例如： What is your present address ? 你现在的住址是什么? The present international situation is excellent . 当前的国际形势一片大好. The professors present at the meeting came from Shandong University . 出席这次会议的教授们来自山东大学. The students present welcomed the decision . 在场的学生都欢迎那个决议. 10. 形容词worth构成的词组作定语时,常常后置.例如： In 1626 the island of Manhattan was bought from local Indians , Natives Americans , for a handful of goods worth $24 . 曼哈顿岛是于1626年从当地的印第安人（即美洲土著人）手中买来的,只花了价值约为24美元的为数不多的货物. A car worth $ 80,000 is not too dear for him to buy . 对他来讲,买一辆价值八万美元的车不算太贵.（划线部分相当于which / that is worth $ 8,000） The tall building worth 200,000 dollars in all has been sold to a foreigner . 总计达20万美元的高楼已经卖给了一位外国人. 11.动词不定式作后置定语表示要做的事（尚未发生）,常与名词有动宾关系,不及物动词加介词. 应注意： 1. 表示将要发生的动作,有时根据句意需要加上介词,这时介词不能省略,因为被修饰的名词是它的宾语. 例The farmers thought of ways to protect their crops. We have only a small cold room to live in. 2. 用于修饰被序数词、最高级或no, all, any等限定的中心词. 例 Marie Curie became the first woman in France to be a university professor. She was the first person in the word to receive two Nobel Prize. He was the best man to do the job. 3. 根据不同的句意,不定式可用主动语态或被动语态. 例 Have you got anything to post? (post的动作执行者是“你”) Have you got anything to be posted?(post的动作执行者是“我”或“别人”) There are some plates to wash (to be washed).

选b
本句只有一个谓语did
assistant用分词短语修饰
此时,是“服侍他的那个助手”,故是主动,用serving
如果是The assistant who served her就对了,可惜没有who,那就选b
那个服侍她的助手没有像她那样穿衣服