Question

The purpose of a controlled experiment is to allow a scientist to isolate and test?

Answer 1

A single variable.

In an experiment we are trying to figure out how one thing (independent) effects another thing (dependent) so we want to keep everything constant. We want that experiment to be controlled so nothing else is effecting our dependent variable. That way we can be 100% sure that any change to our dependent is due to our independent variable. Hope that makes sense