While testing the effect of thread thickness on the load it can bear before breaking, which is the independent variable?

Correct answer: Thickness of thread

The direct answer is A: thickness of the thread. The independent variable is the factor deliberately changed to study its effect. Here, the investigation asks how thread thickness affects the load supported before breaking, so thickness is changed from one trial to another. The supported load is the observed result and is therefore the dependent variable. Option A is correct. Option B, supported load, is dependent because it is measured at breaking. Option C, thread length, should normally be kept constant as a controlled variable unless the investigation is specifically about length. Option D, material, should also be kept the same for a fair comparison; changing it would introduce another factor. The experiment must change thickness while keeping other important conditions alike. Exam cue: “effect of X on Y” usually means X is independent and Y is dependent.