According to the Lewis concept, an acid is a species that can accept an electron pair . $\text{BF}_3$ is an electron-deficient molecule with an incomplete octet around the boron atom, making it a strong Lewis acid . $\text{SnCl}_2$ (Stannous chloride) and $\text{SnCl}_4$ (Stannic chloride) both have empty orbitals (p and d orbitals, respectively) in the valence shell of the central tin ($\text{Sn}$) atom. Therefore, they can expand their octets and accept electron pairs to act as Lewis acids (e.g., $\text{SnCl}_4$ can accept two $\text{Cl}^-$ ions to form $[\text{SnCl}_6]^{2-}$). Thus, all three species behave as Lewis acids.