Final answer:
Option A: Being available to deliver keys and resolve issues after closing is the responsibility involved.
Step-by-step explanation:
The responsibility that involves delivering keys to the buyers and resolving issues after closing is the correct answer is option A: Being available.
After closing, the buyer needs the keys to their newly purchased property. The responsibility of the seller is to be available to deliver the keys promptly and also address any issues that may arise after the closing.