Because of the way the CISSP exam is structured, there is only one correct answer for each question; many times one or two of the answers will be obviously incorrect and two of the answers will be plausible. Take the time to reread the question. Words such as sometimes, not, always, and best can make a big difference when choosing the correct answer. Unless the answer leaps out at you, begin the process of answering by eliminating those answers that are most obviously wrong.
Almost always, at least one answer out of the possible choices for a question can be eliminated immediately because it matches one of these conditions:
After you eliminate all answers that are obviously wrong, you can apply your retained knowledge to eliminate further answers. Look for items that sound correct but refer to actions, commands, or features that are not present or not available in the situation that the question describes.
If you're still faced with a blind guess among two or more potentially correct answers, reread the question. Try to picture how each of the possible remaining answers would alter the situation.
Only when you've exhausted your ability to eliminate answers but remain unclear about which of the remaining possibilities is correct should you guess at an answer. An unanswered question offers you no points, but guessing gives you at least some chance of getting a question right; just don't be too hasty when making a blind guess!