这是一个有意思的问题，我在手头的资料中通过检索，在2013年的高考英语全国三卷的短文改错中查到一个高度类似的例子。

下面是当年高考题短文改错的原题（其中有错）：

In my family, there are three people. My father is hardworking but goes to work in the field every day. He isn’t good at talk but he gets on well with other people. My mother is very much kind and is friendly to everybody. So when I have the problem I will turn to her for help. My friends say I am clever. When the teacher asks us very difficulty questions, I’ll think quickly and stand to answer. At home my father often thinks I’m silly. He said if I decide to do something, it takes him much times to stop me. This is how I need to improve in the future.

下面是当年高考题短文改错题经过改错后的短文（加粗部分为参考答案提供的“给分答案”）：

In my family, there are three people. My father is hardworking and goes to work in the field every day. He isn’t good at talking but he gets on well with other people. My mother is very kind and is friendly to everybody. So when I have a problem I will turn to her for help. My friends say I am clever. When the teacher asks us very difficult questions, I’ll think quickly and stand up to answer. At home my father often thinks I’m silly. He says if I decide to do something, it takes him much time to stop me. This is where / what I need to improve in the future.

最后一句的This is how I need to improve in the future.改正的方法是改为：

This is where / what I need to improve in the future.

    我觉得选 where 和 what 都可以：

    1. 若选 where，其中的 improve 为不及物动词。where 的意思是“......的地方”,例如：This is where I was born. 这是我出生的地方。（外研社、建宏《英汉多功能词典》，1997年1月第1版） 

    2. 若选 what，其中的 improve 为及物动词（what用作 improve 的宾语）。

个人觉得还是用B what 更合适些。which引导非限定性定语从句，修饰前面整句，which is what he should improve in the future 更好些，尤其在考试题里。我手头没有更多例句作依据，仅是凭个人感觉。其他老师如有权威例句能补充更好，本人非常感谢。