Solve 16th one and give proper explanation
Q.16. $BC{l}_3$ does not exist as a dimer but $B{H}_3$ exists as $B_2{H}_6$ because
(a) $C{l}_2$ is more electronegative than hydrogen
(b) large size of chlorine atom does not fit between small sized boron atoms, while small-sized hydrogen atoms occupy the space between boron atoms
(c) there is $p\pi -d\pi$ back bonding in $BC{l}_3$
(d) both (b) and (c)

BH₃ is an electron deficient compound which has an empty p orbital or we can say 6 electron in its outermost shell. As the size of Hydrogen is small therefore it facilitates the dimerisation of BH₃ through 3 entered 2 electron bond. This dimerisation results in the formation of B₂H₆ ( diborane ), where the single hydrogen is shared between two boron atoms.
On the other hand, BCl₃ does not dimeries due to its small size. As the size of boron is small therefore it is unable to accommodate 4 large chlorine atoms at the same time.

Thus, correct option is (d).