Translation:The Fruits are rotten
Sorry, I was probably quite misleading and I know absolutely nothing about Swahili. I am just comparing this with the situation in some Indo-European languages I know, like Latin and Russian. Translating it by the past would be wrong (I was counted wrong on it as well), because they probably have some other way of expressing something that was rotten at some past point. I am just assuming that works like a lot of languages in which something like "he is tired" is translated by a past tense verb.
The translation is literally "the fruits have become rotten", but as that is the normal way of expressing that they are now rotten, it is usually translated, "The fruits are rotten." Of course in English we would rarely say that the FRUITS are rotten, unless we were talking about different kinds of fruit. Even then, we would usually say the FRUIT is rotten. Fruit can be singular or plural.
My understanding is that 'kuoza' means 'to become rotten' rather than 'to rot'. Thus 'yameoza' means 'they have become rotten' rather than 'they have rotted'. So 'yameoza' best translates as 'they are rotten'. it is a subtle distinction, but for me it explains the preferred answer. Similarly, 'kuchoka' means 'to become tired' rather 'to be tired', so 'nimechoka' translates best as 'I am tired.' All IMHO.