Explanation
Correct answer: A.
Choice A is the best answer. The previous sentence tells us how economists in India "use" a certain tool, while this sentence describes general facts about that tool. To express general facts (and also to match the simple present tense of "use"), we should use the simple present tense form "measures."
Why the other choices are wrong
Choice B
Choice B is incorrect. This choice uses the past perfect tense, but the previous sentence tells us that the tool is currently used to measure things, so the past tense doesn’t make sense for this verb.
Choice C
Choice C is incorrect. This choice uses the future perfect conditional tense, but the previous sentence tells us that the tool is currently used to measure things, so the future tense doesn’t make sense for this verb.
Choice D
Choice D is incorrect. This choice uses the future perfect continuous tense, but the previous sentence tells us that the tool is currently used to measure things, so the future tense doesn’t make sense for this verb.