As a further discussion to BugDozer's node, you can also perform 'statistical analysis' on the available answers to give yourself a better chance. Note that this method does not guarantee results, only that it is likely to give you a higher percentage chance of obtaining the correct answer. You still need to know a bit about your subject material to succeed.

Step 1:
Eliminate the obviously wrong answers (there is usually one in every question)

Step 2:
Perform statistical analysis on the answers by looking for common threads in the answers. Consider this example:

Say there are four answers:

1. A, B, C, D
2. A, C, D
3. B, C
4. A
First, 'A' is in three out of the four options. Therefore I eliminate answer 3. I can also eliminate answer 4 because three out of the four answers had B/C/D. That leaves answer 1 and 2. Both answers 1 and 2 have A, C, and D so B becomes the deciding factor. Now B appears in answer 1 and 3, but C appears in answers 1,2 and 3. Therefore I would select answer 2 as my answer.

I've used this technique in 6 years of uni and while it isn't foolproof, I have found it to be a much better option than guesing. I 'proved' this technique to a friend who was doing a test national chemistry exam. With minimal knowledge of chemistry, I scored 14 out of 20 simply using this method. That's 70% accuracy compared with a theoretical 20% accuracy with guessing (5 options per question).

Obviously, the more you do know about the topic, the easier it is to eliminate the obvious wrong answer and the easier it is to pick the correct options. Like I said before, this is but one technique to help improve your chances - there are no guarantees.