This is an assumption critical reasoning question, with a very weak argument presented. The author states that unemployment has risen less during her time in office than it did during her predecessor's time, and therefore her economic policies must be more effective. It is a horrible argument, because there are thousands of possible factors that could have contributed to this other than the economic policies that she has set. So I am going into the answer choices looking for an answer choice that rules out some other possible reasons.
Answer choice A weakens the argument if anything. If the population dropped significantly, then maybe it was easier for people to find jobs (which could have stayed more constant), regardless of economic policy.
Answer choice B is out of scope, as it is focused on the national economy, and also would weaken the argument if anything by showing generally better economic conditions during the time that she was in office over her predecessor's time.
Answer choice C is correct because it rules out a couple of other possible factors. If you negate this (Key socioeconomic variables...are NOT comparable for each administration) and plug it back into the argument, you can see that the argument disintegrates - which is what you are looking for on assumption critical reasoning questions.
Answer choice D would strengthen the argument, but it isn't a necessary assumption. There could have been many smaller changes that the administration made, or blunders that it avoided, rather than key policy changes that it made.
Answer choice E may also strengthen the argument (if we assume that she was responsible for implementing those tax incentives), but it is not a required assumption because she could have done something else with economic policy that reduced the growth in unemployment and the argument would still hold.
I hope this helps!