Oliver Cowdery initially wrote the plural men in 𝓞, then virtually immediately corrected it to man by overwriting the en with an (there is no change in the level of ink flow). The use of the singular man is common in the Book of Mormon text whenever man is equivalent to ‘mankind’. See, for instance, the nearby discussion under Alma 40:8. In fact, here in Alma 42, we have one passage containing several instances of man with the meaning ‘mankind’ that are consistently referred to with plural pronouns:
Here in Alma 42:4, either the singular man or the plural men will work in theory; thus we accept the corrected reading in 𝓞, the singular man with the meaning ‘mankind’.
Summary: Accept in Alma 42:4 the corrected man in 𝓞: “there was a time granted unto man to repent”; the correction in 𝓞 was virtually immediate.