The atom with atomic number 20 is most likely to combine chemically with the atom whose atomic number is

Atomic number 20 will be distributed in electron shell as 2,8,8,2.
Its valency is 2 . It will loose this 2 electron to gain stability and noble gas configuration. If atomic number 11 is distributed in electron shell it will be 2,8,1.
Atom with valence electron 1 will loose this atom to gain stability. Hence, both ${}_{20}\mathrm{X}\&{}_{11}\mathrm{X}$ will gain nothing i.e. will not gain stable electronic configuration after combining with each other.

An atom with atomic number 16 has electronic configuration as 2, 8, 6.It will gain 2 electron from ${}_{20}\mathrm{X}$ and complete its octet. Thus atom having atomic number 20 will combine most likely with atom having atomic number 16 as both will gain stable electronic configuration.

Hence, the correct option is option (D) 16.