Your answer would be B. He's talking about not being able to prove his age because he hasn't never seen a real record of his birth or anything else that would give him an idea of how old he is, so no real way to actually prove it.
Do you have to explain these or do you have to write a story
By explaining problems and giving solutions! this is the best fit because the other two might make sense at a first glance, but only explaining problems and solutions would be an effective way to teach people how to set up an aquarium by giving them examples of what to do and what not to do whereas the others are just him talking about his aquarium.
