Which of the following is not a Lewis acid?

Which of the following can give $OH^{-}$ ions?

The values of dissociation constants $(K_a)$ of some acids (at $25\,^{\circ}C$) are as follows. Indicate which is the strongest acid in water:
$(A)$ $1.4 \times 10^{-2}$
$(B)$ $1.6 \times 10^{-4}$
$(C)$ $4.4 \times 10^{-10}$
$(D)$ $4.3 \times 10^{-7}$

The theory of ionization was given by:

Acidity of $BF_3$ can be explained on which of the following concepts?

The conjugate base of $NH_2^-$ is