Since this rule applies somewhat differently to the SAT and ACT, I’m going to discuss each test separately.
SAT (Fixing Sentences)
Always check answers in order of length, starting with the shortest one. In general, the correct answer will be the shortest answer that is grammatically correct. While the right answer won’t always be the shortest option (although it often will be), it’s highly unlikely to be one of the longer options – at least until the last few questions of the section, where all bets are off in terms of length. Why? Because short, clear constructions are usually more effective than long, awkward ones, whether you’re dealing with the SAT or not.
When you are given a phrase rewritten several ways, all of which are grammatically correct, the shortest one will virtually always be right.
This rule applies only to general non-grammar question, NOT ones that require you to give a sentence a particular focus (e.g.” Which of the following most effectively emphasizes the author’s surprise at discovering a frog in her living room?”) Why? The longer options are generally either redundant or contain irrelevant information.
Incorrect: I decided to ask my mother the question, which required an answer.
Correct: I decided to ask my mother.
The only thing that one can ask is a question; and a question, by definition, requires an answer, so the inclusion of this information is unnecessary. While I wouldn’t ever advocate choosing an answer without reading it first, this rule is pretty foolproof. If you’re pressed for time, just go for the shortest one, and you’ll usually be right.