Option A - Incorrect
The topic of interest is about if the cake has turned moldy or not based on the number of days it has spent. So, the question about Alex's refrigerator being clean or not was discussed. Therefore, this option does not, in any way, strengthens the statement. This option is thus incorrect.
Option B - Incorrect
A blown fuse? That's out of the point. Nothing a such is discussed, in fact, electricity being a factor that culminated to the moldy cake was not discussed. This proves error and does not in any way strengthen the statement.
Option C - Correct
Yes, there was a cake in Alex's refrigerator, and it has remained there ever since. This is because the question says it is at least one week old. This shows particular that a cake is in there since it has been baked.
Option D - Incorrect
A recent study? That is not a part of the discussion in the full statement above. A survey or something a such was not deliberated upon, bring that into this choice is nothing but off point and tends towards invalidity. This option is wrong
Option E - Incorrect
A particular day of the week was not discussed as the day the cake was baked, and it does point us to strength this as however shifted drastically from the actual point of sense. Incorrect option