Step 1: Understanding the Concept:
"Prune" means to trim or cut back branches or reduce something by removing unwanted parts.
Step 2: Detailed Explanation:
In gardening, pruning reduces the size of a plant. By extension, "prune" means to reduce or cut down. "Lend" means to give temporarily. "Expand" means to increase (opposite). "Prolong" means to extend in time.
Step 3: Final Answer:
Thus, "Reduce" is the correct synonym.