The correct option is A (A) is correct; (R) is correct and (R) correctly explains (A).
BF3 is a weaker Lewis than BCl3 because of the more effective back-bonding in boron trifluoride. B and F belong to the same period and this the back-bonding (donation of a lone pair on F into the pure, unhybridized 2p orbital of B. As for BCl3, the back-bonding is not as effective because of the difference in energies of the pure 2p orbital of B and the orbital occupied by the lone pairs of Cl. Thus, BF3 is a weaker Lewis acid.
Thus, both (A) and (R) are correct and the latter accurately explains why (A) is true!