When we say that an experiment is controlled, we mean that

The correct answer is everything is held constant except for one variable

Controlled experiment is the scientific experiment that is carried out under conditions that can be controlled. It is done in two groups or samples where they are compared for the hypothesis test, being the experimental group and the control group.

The control group will not receive any intervention and aims to be the comparative element with the experimental group. The experimental group is the one where the variable is tested.