Fill in the blank using the appropriate verb form:
Can you wait for me? It _________ very long.

The sentence "Can you wait for me? It _________ very long." is discussing a future event and requires a negative form of the future tense. The most suitable option is "won't be," which correctly completes the sentence by indicating that the wait will not be very long.

Option B: haven't be
This option is incorrect. "Haven't be" is not a grammatically correct form for this context. The correct form should use the future tense "won't be."

Option C: aren't be
This option is incorrect. "Aren't be" is not grammatically correct in this context. The correct form should use the future tense "won't be."

Option D: didn't be
This option is incorrect. "Didn't be" is not appropriate for expressing a future action. The correct form should use the future tense "won't be."

The correct answer is Option A: won't be, which accurately conveys the future negative tense and completes the sentence in a grammatically correct manner.