The correct option is A Cl−
According to Bronsted-Lowry and Lewis Concept of acid and bases:
Species (molecule or ion) which has a tendency to accept a proton can act as a Bronsted Lowry base.
Species which has a lone pair of electrons available for donation can act as a Lewis base.
BF3 has a empty p orbital, and can accept a lone pair of elecron to act as a lewis acid, not a lewis base.
CO cannot accept a proton and hence cannot act as a Bronsted base. It can act as a Lewis base, as the lone pair on carbon can donates it's electrons, (for example forming transition metal coordination complexes).
While Cl− accepts a proton to form HCl. It also has a lone pair of electrons available for donation. So, it can act both as a Bronsted Lowry base as well as Lewis base.