The principle of opportunity cost states that if you have more of one thing you must have less of another true or false

Hi there. The way this question is stated is going to need some input from you to infer what was meant by the language.

This answer would be True. Because opportunity cost means more of one and less of another in simple terms, since in an economy we cannot utilize all of our resources at one time.

But I would caution that the language is in the simplest of terms! Since a more thorough definition would be that opportunity cost is the amount lost by not taking the next-best alternative. If your teacher and lesson usually describe opportunity cost and other terms simply, then I would say to go for it and choose True.
But if your lesson seems to be strict on definitions, then maybe that isn't the best definition. I would say it is good enough though because although simple, it does encompass the whole idea.

I hope this helps!