Help me solve this problem

B is correct.

He is very confident he will pass the test. He is extremely confident.

He is so confident that he won't even study for the test.
I disagree. In my opinion, he is (much) too confident. (I think he is overconfident.) People who are too confident and do not prepare for their tests are often disappointed.

I hope he won't be disappointed. He seems too confident for his own good.

I hope he won't be disappointed. He seems much too confident for his own good

You use too in order to indicate that there is a greater amount or degree of something than is desirable, necessary, or acceptable.
Leather jeans that are too big will make you look l

Well,I still can't discriminate between extremely and much too. What's the difference about meanings between them?

As Yoong Liat said,

Yoong LiatYou use too in order to indicate that there is a greater amount or degree of something than is desirable, necessary, or acceptable.

There is an excess of confidence.

extremely is just a state on the end of a spectrum of values. (eg
It is hot today. (30 degrees C)
It is extremely hot today (45 degrees C)

much too includes my opinion on that state.

It is much too hot today to go for a long walk.