The phrase "to complain" refers to an uncountable quantity, so we use "much."
The correct sentence should be:
"He does not have much to complain about."
Thus, the correct answer is "much" (Option A).
Arun’s present age in years is 40% of Barun’s. In another few years, Arun’s age will be half of Barun’s. By what percentage will Barun’s age increase during this period?