A. Despite the gains made by American women in recent decades, the median salary of women is still only about 78% of that of a man. ----
Median salary of women being compared to that of man is not an equal comparison. B. Despite the gain made by American women in this recent decade, the median salary of a woman is still only about 78% of a man’s. -----
use of the singular gain and the singular decade changes the meaning C. Despite the gain made by American women in recent decades, the median salary of a woman is still only about 78% of a man.—
the singular gain is not correct. More importantly the comparison of a median salary is done with the man himself and not his median salary D. Despite the gains made by American women in recent decades, the median salary of a woman is still only about 78% of that of a man’s. –
78% of that of a man’s is double possessive and redundant. E. Despite the gains made by American women in recent decades, the median salary of a woman is still only about 78% of a man’s.-----
Best