Explanation

Lewis acids are electron-pair acceptors: Al3+ and Co3+ are cations that can accept electron pairs, and BF3 is electron-deficient at boron and readily accepts a pair. Options (b), (c) and (d) each contain Lewis bases — NH3, OH-, H2O or F- — which donate electron pairs rather than accepting them. They typically form coordinate bonds by accepting a lone pair into a vacant orbital.