Question

I didn't buy .... mangoes​

Answer 1

Answer:

I didn't buy the mangoes

Answer 2

There can be various answers for this which can be used in different contexts.

1. I didn't buy any mangoes. - no mangoes were bought.
2. I didn't buy the mangoes. - the person did not buy the mangoes.
3. I didn't buy these mangoes. - the person did not buy the Mango which was there.
4. I didn't buy his/her/their mangoes. - the person did not buy mangoes from him/her/them.

The correct answer would be what suits your sentence best.