O.K. :) Duolingo in its style ha, ha ! Here they want " havy rain" and somewhere else - " rainy season ".
Why does this question give "mvua kubwa" as an acceptable translation of "heavy rain," but others do not accept it?
Duolingo is not being specific in many of the questions. Mvua Kubwa literally means a heavy rain fall. As in, it is raining heavily right now. The other translations are (usually) asking for the season which has heavy rainfalls, which is masika. So masika is the season, and mvua kubwa is the rain fall.