------superstitious beliesfs about mandrake plant.A people h

选择：c. people have long had

解析：
People have long had superstitious beliefs about the mandrake plant. 【参考译文：人们（迄今）始终有关于关曼德拉植物的迷信】
句中的 long 是副词,意思是“长期地/始终”,此处是中位状语.按语法规定,中位状语只能放在助动词 have 和动词 had 之间.

注：
have had —— 现在完成时,表示从过去一直到现在
superstitious belief ——迷信
mandrake plant —— 一种提取镇静剂的植物,这种植物曾被认为具有魔力.

I don't now……

请看下面的解释.
供楼主参考.
--------------------------
Abolish applies only to doing away with conditions,practices,or regulations,not material things or persons:
Abolish 仅指消除状况、行为或条令,而不是物质或人
Exterminate suggests total destruction,as of living things,by a deliberate,selective method:
Exterminate 指用故意、有选择性的方式完全毁坏,如生物
Extinguish means to put out a flame or something likened to a flame:
Extinguish 指扑灭诸如火之类的事物
Extirpate suggests effective destruction by removing roots or entrenched causes:
Extirpate 指用除去根或其他稳固的根源进行有效的破坏
Eradicate shares the connotations of extirpate but stresses the resistance to dislodgment offered by the object:
Eradicate 有Extirpate 的含义,但强调物体所产生的抵抗
Obliterate means to destroy so as to leave no trace:
Obliterate 指不留痕迹地破坏

这个句子来自Helen Keller的Three days to see（假如给我三天光明）,已经有人翻译了：
我的童年时代仿佛笼罩在一片金色雾霭中,要拨开这片薄幕,我踌躇不决.（译者：鹏鑫）
可惜漏译了as it were （一定程度上）和superstitious（迷信的；没有科学根据的）
改译如下：
我的童年笼罩在金色雾霭般的薄幕中.要拨开这片薄幕,我有点顾忌而犹豫不决.

to tell the truth,everyone is superstitious to a certain extent
说实话.
如果填后面两个,就会在一个句子里有两个动词,出现错误
第一个没有两个动词,但是如果是第一个主语应该是I
而这里主语是everyone

副词修饰动词位置较灵活,但不置于助动词前.

the second one is wrong in my opinion