Why can't this mean, "we eat bananas and the apple"?
Because it's not "हम केले और एक सेब खाते हैं " Context. I feel if it's we eating just one apple, the number would be more emphasized. Since it is we, one would more likely assume more than one apple.
It is because kala comes first and sab doest and kala means bananas and sab means apples
I think "we eat apples and bananas" should be right as well. It does not change the meaning of the sentence.
The order is wrong in your translation. The bananas come first than apples.
That's right. But if DL accepted it, then it can't test whether you know the difference between apples and bananas.
When do we write kaathe kaatha kaathi hain?
Basic Hindi conjugation
खाता - masculine singular
खाती - feminine
खाते - masculine (or mixed gender) plural and also for use of masculine singular with high respect (तुम).
Why can't it mean apples and bannas
Because it clearly says bananas and apples. केले is bananas, सेब is apples
It can't because kala comes first and not sab
Why can we eat banana and the Apple I have doubt