PA understands the club have taken up that option, extending the defenders contract until the end of the 2020/21 season.
Fosu-Mensah has made 21 appearances for United in all competitions, but the last of those came in May 2017.
The Holland international had season-long loan spells at Crystal Palace and Fulham, with a knee injury preventing playing for Uniteds first-team before the coronavirus saw the 2019/20 season suspended.