When all else fails -- when you've employed the key word tactic, when you've eliminated as many choices as possible, when you've made sure to eliminate those words that make the flow of the sentence go awry -- what can you do? "Listen" to the sentence; perhaps even read it out lout. As with most languages, people have the innate ability to detect when a sentence or parts of a sentence are grammatically or stylistically correct. Does the sentence sound awkward? Do you stumble over the words or do they flow freely? If the flow of the sentence is choppy or if something does not sound right, then chances are that something is not right. Consider the two following sentences; can you spot which one of the two is correct and which one is not?
The boisterous laughter of the swelling crowd deafened our senses.
The trepidity laughter of the budding crowd deafened our senses.
If you don't know the meanings of the words boisterous, trepidity, swelling, or budding, then first consider each sentence, one word at a time. "boisterous laughter" sounds okay, as so does "swelling crowd". The fragment "trepidity laughter", however, sounds odd, and in fact, trepidity, which means fear or distress, does not fit the meaning of the sentence at all, both in meaning and in grammar. The key fact to note here is that in addition to having to know your vocabulary, you'll sometimes need to rely on your intuition to arrive at the right answer, and it's only after you practice and hone in your so-called inner-ear, that you'll be able to zip right through the sentence completion questions.