Answer:
The correct option is a) "decrease the amount of GTP to inhibit microtubule elongation and block secretion"
Step-by-step explanation:
Since microtubules play a critical role in transporting secretory vesicles, preventing it from elongating will keep it from performing this function. Completely eliminating the microtubules from the cell as suggested in option d will harm the cells and other processes in it because the microtubules also have an integral part to play in maintaining cell shape & structure.
Hope that answers the question, have a great day!