What is the best conclusion that can be drawn from this graph?

The answer to your question is the first option: The social reforms introduced by Khrushchev were not working.

Explanation:

The graphs shows a gradual decline in growth. It slowly by every year went lower and lower in growth. Which means that his reforms weren't working.

Why not B? This answer has nothing to do with the question. Its there just to confuse you. Your question says nothing about Brezhnev.

Why not C? Same reason why B isn't correct.

Why not D? His income growth didn't increase but instead decreased as we see in the graph. Each year by year it went down lower.

Conclusion:

Khrushchev wasn't able to profit in income (increase his income), so his social reforms weren't working.

Have a nice day :3