We have the table with percentages (the results of democratic candidates in various cities and on different levels). The task is to determine whether the given statements can tell us why the numbers are such and not the other.
For example, if statement A is a fact, it explains why the percentages in "Governor" column are lower than percentages in Presidential and US Congress columns. If there are two major candidates in an election, most of the votes will split between them, let's say 60% and 35%, or 55% and 42%, etc. However, if there are three major candidates, most of the votes will split between the three, for example 30%, 30%, 35%. Thus even the winner in a three-split can easily have lower percentage than a looser in a two-split.
We see that in these elections people like democratic party, so the lower numbers in the Governor column can be reasonably explained by statement A.
Note, that there can be other reasonable explanations. For example voters might simply not like the governor candidate. However we do not require statement A to be the only possible explanation.
Statement B is simply irrelevant on its own.
Statement C, on its own, works against the numbers in the table. It would explain, if the numbers were different, but not the given percentages.