On a text completion question you should attempt to mentally fill in the missing word or words before you look at the answer choices. This is not because you want to guess the right answer (which, accidentally you might), but because you want to actively think about the questions. Most often you'll have at least a vague idea of the main point of the passage and so you'll find that your solution to the missing word or words may be a synonym for one of the answer choices. For example, consider the following sentence completion question:
In an attempt to _______ the swan so that it could be safely relocated to an animal sanctuary, the warden injected the animal with a tranquilizer, after which the animal slept for over three hours.
Even without looking at the word choices, can you fill in the blank? First, point out the key facts about the sentence: a swan, some sort of an injection, a tranquilizer, and sleep. What do these terms imply? And what does a warden have to do with all this? As you can already imagine, the key phrase, "In an attempt" creates a relationship between the first part of the sentence and the second, and so why would the tranquilizer be administered? You should be able to ration that a tranquilizer is given to calm or to put to sleep, and in fact that's what the second part of the sentence is stating. So, the first blank in the sentence should be a synonym for sleep or calm.
And so now we look at the word choices:
Do any of the answer choices resemble the terms sleep or calm? Enliven means the opposite of sleep, so eliminate that choice. Mediate means to reduce or to arbitrate, and that doesn't make sense. Sedate means to put to sleep, and that's a great match, but make sure that there are no better answers. Secure means to fasten, and that's a valid answer, but sedate is much better, so eliminate secure. Club means to hit, and although that's a valid verb for this sentence, it does not follow with the meaning of the sentence because you wouldn't want to hit something which you want to transport to a sanctuary, which is a place of safe haven. So alas, the third choice, sedate, is the best answer.