Ability scores improve every rank which is a multiple of 4, i.e. at 4th, 8th, 12th rank, etc. The player decides which ability score is improved.
Variations: Ability scores improvement at a different rate, e.g. every rank which is a multiple of 3 instead of 4.
Ability scores improve every rank which is a multiple of 4, i.e. at 4th, 8th, 12th rank, etc. Roll d20 and refer to the table to determine which ability increases.