When you were young, . . . . . . you climb a tree?

The correct answer is B: could.
Let's understand why:

Understanding the Sentence:
The sentence "When you were young, . . . . . . you climb a tree?" is asking about your ability to climb a tree in the past.

Why 'could' is Correct:
* 'Could' indicates past ability. It means you had the ability or were able to do something in the past.
For example: "When I was young, I could run very fast."

Why the other options are incorrect:
* 'Did' is used for asking about actions or events that happened in the past, not abilities.
It doesn't fit grammatically in this case.
* 'How' is used for asking about the manner in which something is done, not the ability.
* 'Should' is used for giving advice or expressing obligation, which doesn't fit the context of the question.

Therefore, "When you were young, could you climb a tree?" is asking about your past ability to climb a tree.