The correct answer is off.
The phrase “put off” means to delay or postpone something. In this case, the speaker is saying that you should not delay doing something until the very last minute. This is because doing so can lead to stress and anxiety, as well as a lower quality of work. It is always better to start working on something early and give yourself plenty of time to complete it.
The other options are not as relevant to the meaning of the sentence. Option A, “over,” could mean to complete something, but it does not have the same meaning of delaying or postponing. Option B, “of,” could mean to stop doing something, but it does not have the same meaning of delaying or postponing. Option C, “on,” could mean to start doing something, but it does not have the same meaning of delaying or postponing.