Question

which can be described as a molecule with residual bonding capacity 
a-N2. b-CH4 c-Nacl d-Becl2

Answer 1

Here you just consider BeCl2.

At room temperature as well as it exists as a dimer (Be2Cl4),  

even it also has an residual bonding capacity  and the BeCl2 is a complete,

neutral molecule,  even it can also form bonds with another molecule,

it is important to attain further stability.