Final answer:
The adjective 'less' in the sentence answers the question 'How much?' as it describes the quantity of effort needed.
Step-by-step explanation:
The adjective in bold in the sentence 'It takes less effort to write essays than you might think' answers the question of 'How much?' Therefore, the correct answer is B. How much?
The word 'less' is comparing the amount of effort needed to write essays, implying that it is not as great as one might expect. This is not about the kind of effort, the specific essays in question, or the number of efforts, but rather the quantity of the effort which is implied to be smaller than expected.