The most common and grammatically correct way to complete the sentence is: "You should avail yourself of the opportunity when it presents itself." This means you should take advantage of the chance to do something when it becomes available. Other common expressions include:
You should act _______ the opportunity presents it self.
Review the verified answer and explanation for this Ministry of National Health Services, Regulations & Coordination (MNHSRC) past-paper question on CampusFin.