he is very young,he knows a loat about how to heahty.A / but

同意一楼.
Athough he is very young,he knows a lot about how to healthy.
尽管 他很年轻,他知道应该怎么保持健康.
He is very young,but he knows a lot about how to healthy.
他很年轻,但是 他知道应该怎么保持健康.
Although 相当于尽管.but 相当于 但是.但在英语中,Although 和 but 不能一起用.

Athough he is very young, he knows a lot about how to healthy.
He is very young, but he knows a lot about how to healthy.

上二句都对，只是but和 Athough 不能连用

原句应该是：______ he is very young, ______ he knows a lot about how to be healthy. 吧
选B
其实
He is very young, but he knows a lot about how to be healthy.
和
Although He is very young, he ...

选B，正常都可以选，但是在这里要是选A，he谓语句首要大写，就是这样，but和Athough不能连用就行，二者选一

Athough he is very young, he knows a lot about how to healthy.也可以是 he is very young, but he knows a lot about how to healthy. although与 but 是虽然但是的意思 但是在英语中不可以同时用 ，与我们的汉语不一样，但是在翻译时可以直接翻译出来

Athough he is very young, he knows a lot about how to be healthy.
He is very young, but he knows a lot about how to be healthy.

答案：B，让步状语从句。他虽然年轻，但是很了解如何保持健康。

Athough he is very young, he knows a lot about how to healthy.
He is very young, but he knows a lot about how to healthy.

楼主，请一定要将题目抄清楚，不要漏了空，不要打错了字，更不要随意改选项。经搜索，发现原题如下：
江苏省昆山市2012届九年级下学期第一次质量测试英语试题_2012 ...
2012年4月24日 ... ______ he is very young, ______ he knows a lot about how to be healthy.
A. Though; but...

but althoug 不能同时用