What is the grammatical function of calculating the area of a triangle

Which is not among the style choices a writer would use, if he or she is trying to establish a formal writing style/

Aids is growing fastest in the poorest countries of africa. these nations don’t have enough doctors, medical labs, or money for medicine. a recent study showed that only a small percentage of hiv-infected children in africa are getting any medical treatment at all. what central idea is emphasized in both sources? the hiv/aids epidemic in africa is serious. the hiv/aids epidemic in south africa is serious. the hiv/aids epidemic among african children is serious. the hiv/aids epidemic among south african children is serious

I’ve given a lot of thought to what one of them should be, and the answer seems clear.