The correct option is D BeCl2
a) The lewis structure of BH3 shows that boron has only three bond pairs and no lone pairs of electrons. Hence, it can accept electrons from a donor to complete its octet and act as a Lewis acid. In B2H6 structure there are two electrons shared between three centres, forming a 3c−2e− bond. Since this is an electron deficient bond, it can still act as a Lewis acid.
b) BeCl2 is also a lewis acid due to the incomplete octet of Be and its ability to accept electron pairs, for which it can accept lone pair of electrons from strong lewis bases. When it polymerises Be has coordinate bonds formed by Cl in a strained polymer and it acts as Lewis acid in presence of a strong Lewis base.
c) In AlCl3, Al is trivalent and hence it accepts a lone pair of electons from a lewis base to complete its octet and act as a lewis acid. But when it dimerises, bridged Cl forms coordinate bond and the octet of Al is complete but it will still act as a lewis acid due to the vacant d orbitals and accept the lone pair when a lewis base is around.
d) BeH2 is also a lewis acid due to the incomplete octet of Be and its ability to accept electron pairs to complete this octet, for which it can accept lone pair of strong lewis bases. When it dimerises as Be2H4, there are two electrons shared between three centres, forming a 3c−2e− electron deficient bond. So, the octet of Be centres is still incomplete and hence act as lewis acid in dimerised form.