Quelle est la lecture normale de cette expression ? "Un à un"

The question asks for the normal reading (pronunciation) of the expression un à un, which means "one by one."

In spoken French, the preposition à (with a grave accent) is simply pronounced /a/. The grave accent distinguishes the preposition à from the verb form a (third person singular of avoir) in writing, but both are pronounced identically: /a/. Therefore, when reading un à un aloud, it sounds exactly like Un/a un - three distinct sounds with no liaison or added consonant between the words.

The other options are incorrect: Un/á/lun inserts a spurious /l/ sound that does not exist in the expression; Un à l'un adds the article l', changing the expression entirely; and Un ã un places a tilde (~) over the a, which is a Portuguese diacritical mark and does not exist in standard French orthography.