Aconversion factor is a ratio of equivalent values used to express the same units in different quantities

C - Variable

Explanation:

Let’s see all the options in detail.

Apparatus: These are the equipment’s used in the experiment. Irrespective of the experiment result, the apparatus remains unchanged from start till the end.

Constant: These are factors that are fixed and does not change throughout the experiment.

Variable: A variable is any factor in an experiment that changes. As per dictionary, variable literally means something that varies i.e. change.  

Hypothesis: This is the underlying explanation in response to which the experiment is being conducted. Also, in an experiment, the hypothesis is not considered a factor at all.

Therefore, the correct answer is C.