Translation:These four trees are starting to yield good apples.
"Začínat" is an imperfective verb. As I understand it, these verbs are USUALLY translated to present continuous in English.
Also, it is better in English to use "starting to bear" rather than "start to bear" in this sentence. I think "start to bear" suggests that some qualifier -- say, "in August" -- should follow the verb.
Can the translation hints be set for the specific exercises, or are they general? I don't think the suggested translation (and even an accepted answer), "carry", is used in this context. Probably everybody has the common sense to figure out that the hint is incorrect, but if it's possible, it should be changed.