What is 40% of 25% of 800?

Apply the percents one after the other. First, $25\%$ of $800$ is $0.25(800)=200$. Then $40\%$ of $200$ is $0.40(200)=80$, choice C. Alternatively, multiply the decimals: $0.40\cdot0.25=0.10$, so the answer is $10\%$ of $800$, which is $80$. Choice D ($200$) stops after the first percent. Choice B ($32$) is $4\%$ of $800$, which comes from misplacing a decimal. Choice A ($8$) is $1\%$ of $800$, a decimal shift of one more place. Always sanity-check: the result must be smaller than $200$.