Answer
A. Disruption of mitotic spindle formation.
Work Step by Step
Only A has something (spindle) involved with the microtubules mentioned in the question and, thus, on which vinblastine could act.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.