Esther, I am not sure of the context in the lesson/exercise, but either sentence can be used in conversations depending on situation.

The first sentence would be ‘je les ai mangées’ (plural) to be grammatically correct. It emphasises you have eaten ‘the mangoes’ - used in the context of knowing what ‘les’ represents in type and number (quantity). 

The 2nd sentence emphasises ‘how many’ of them - used in the case of knowing what ‘them’ is in type, but wanting or needing to specify the number ‘of them’.  

Your assumption about the significance of the underlining in a question or lesson seems likely correct.

Que diriez vous a "Je les ai mangée" ?

Luis, I am not sure what you are asking. Is your question seeking more explanation of the usage of the phrase per se, or are you questioning the grammar ? Both are correctly covered in the previous response, but can clarify if you have a specific question.

Thanks Maarten