"How do you do"is a very ___ question which you don't have to answer.

B
看后半句：you don't have to answer
既然后半句否定，就意味着“有理由不去做某事”
按照常理，不应该不去回答一个常见的问题，所以选B

还有，英语题目有的比较冷门而且较难解释，这时应该联系语境。打破思维定势，大熟词在一些比较怪的题目中通常不会选。。。。

A，这是一句常见的问候

B一般问题都要答案,它却you don't have to answer,所以unusual

B，different from the usual ones.

B] unusual
I think should choose B

Generally,a question should be answered. But here the question"How do you do?" is not a common question. It is used to greet others. It is uncommon.The question has a fixed answer,"How do you do?"
so here you should choose B, it is an unusual question you do have to answer(The Chinese meaning of the sentence is:它是一个没有必要回答的特殊的一个问句。)