What type of variables are held constant in an experiment?

Answer 1

Trick question? If a parameter is held constant, then by definition that parameter is not a variable. Depending on the experiment - that is, what you are trying to measure or detect, and what thing(s) you change in order to manifest a result; just about any physical property can be made to be variable or constant, with one famous exception - and you know what that is, right? (Hint: e = mc^2).