This kind of error in reasoning question is asking us to phrase the flaw in a specific way. It is probably best to understand the flaw in your own words first.
In this case, the argument tells us that if one supports the tax plan they will not be elected, and that anyone who understands economics will not support the tax plan, so therefore the winner must be someone who understands economics.
This is flawed because it confuses a sufficient condition for a necessary one. Understanding economics guarantees opposing the plan, but maybe someone might oppose the plan for other reasons.
We are then asked to find that the argument is flawed since it "ignores the possibility that..."
Looking at how we phrased the flaw, D looks like a match. If someone opposed the plan for other reasons, they might not understand economics. Thus it is possible someone opposes the plan but doesn't understand economics, so D is correct.
