There are NO stupid questions here.
This is a type of GAMSAT question that requires simple reasoning.
The question asks why men have a higher mortality rate than women if diagnosed with melanoma, regardless of disease stage. Now you have to take note of the phrase 'most likely explanation'. This suggests that you are to look for the option that does not need any outside knowledge or medical research background. You only need to use some cause and effect reasoning:
If a person does not visit a doctor (or is less likely to do so), then there would be no medical diagnosis. This means that the mortality rate among males is not dependent on the stage of the melanoma disease.
I hope this makes sense.