Im am quite confused as the logic for why C is incorrect. It states that this is because there are now two independent variables which means that you cannot differentiate the two. Which I totally agree with normally. However, if you are performing an experiment and you separate your groups into old and young, and then perform the experimental conditions within each group (high/low testosterone for old and young) then would this mean that you are now "controlling" in a sense for age? I get it if it you split into old high testosterone and young low testosterone. But that isn't what is being done (and the answer explanation even states this).

Thanks for helping me!

Hmm, so I get that you can't compare between the age groups, because that would result in confounding the age variable. But, could you also just compare within an age group and not across them? Then you wouldn't be confounding for age, you would actually be controlling for age, which is how I interpreted the answer.