Neon and Argon are inert elements (also noble gases) with full outermost electron shell so they can exist as atoms --> diagram D.

Oxygen exists as O2 and Nitrogen N2 molecules (they bond to themselves and exists as pairs of atoms) --> diagram F

NH3 --> 1 Nitrogen bonds to 3 Hydrogen --> diagram C

CH4 --> 1 Carbon bonds to 4 Hydrogen --> diagram A


The key point to this question is to count the atoms in a molecule and take note that in the diagram, the same atom is represented by the same kind of circle/shading.