This is a classic one.

So, research supports that there is a connection between consumption of trans-fat and heart disease. And people started calling out for its(trans-fat) use.

We need a positive tone that trans-fats will be banned.

The next sentence is saying that due to outcry it is unlikely that lawmakers would avoid banning it(trans-fat).

Double negative makes the sentence positive. So the lawmakers would surely ban trans-fats.

Answer A

After research began to overwhelmingly support the connection between the consumption of transfats and heart disease and the city’s electorate began an outcry against their use, it seemed (i) ___________that lawmakers would refrain from banning them from restaurants.

The word "seemed" here takes the meaning "it gives the impression".

Therefore, given that the connection between consumption of transfats and heart disease has been established and the city's electorate began an outcry against its use, we can expect the lawmakers to ban them from restaurants. But we have the phrase "refrain from" which negates the expected meaning. So this in turn has to be negated to give the expected meaning - "we can expect the lawmakers to ban them from restaurants" - and the only way this can be done is by using another negative, and hence improbable is the correct answer.

The word "seemed" here does not indicate a contrast.

The word "seem" has different meanings in different contexts, and clearly the GRE is laying greater emphasis on understanding the meaning of the sentence and solving it, rather than through using rigid rules.