It depends on the answer key.
Guessing 'C' all the way down and expecting 25% assumes that the distribution of answers is roughly equivalent across the whole test. This is an extra variable you can't account for without knowing the answers.
Whereas, if you pick a random answer for each question, you will answer 25% correctly, assuming an infinite number of questions. However this strategy has error bars that grow as the number of questions gets smaller.(ie: you are far more likely to end up with more or fewer than 25% correct in practice)
The best strategy for any given multiple choice question that you don't know the answer, is to eliminate options that are definitely incorrect, leading to a much higher chance of success with a random guess.
Or just study the content more, as breadsmasher suggested.