The correct option is
B have improved
The speaker 'I' asks the listener 'you' the first question: what he thinks about his English. The sentence is in simple present tense, and asks for the listener's (you) current opinion.
The second question asks for a confirmation, whether 'I' has improved.
So, the action 'to improve' would have just completed, hence 'I' is asking for the opinion of 'you'.
Option B: 'Have improved' is in present perfect tense. This tense expresses an action that has just been completed. The tense agrees with the given sentence: 'I' asks 'you' if 'I' has improved in English. Hence option B is correct.
Option A: 'Will improve' refers to a future action. If the action is yet to happen, 'I' wouldn't ask 'you' for his opinion for an action not yet happened.
Option C: 'Am improving' refers to a continuous action.
'i' asks 'you' for his improvement so far, not if he's still improving. Hence this tense is not appropriate.
Option D: 'Have been improving' is in present perfect continuous tense. It expresses an action that started in the past and is continuing till the present. Similar to option C, the person 'I' asks the listener 'you' for his opinion on his current English till date, which refers to the present perfect tense.
Hence options A, C and D do not contain the appropriate tenses and they're incorrect.