In the $3D$ representation of an organic molecule,which model indicates atomic size?

Which of the following will have the highest enol content?

The correctly matched pairs of the following are given by the sets $:$
$(A)$ Neopentane $-$ position isomer of isopentane
$(B)$ Isopentane $-$ three primary carbon atoms
$(C)$ Isobutylene $-$ no secondary carbon
$(D)$ Isobutane $-$ two primary carbons

Cyclohexa-$1,3$-diene contains . . . . . . vinylic and . . . . . . allylic hydrogen atoms?

Fill in the blanks:
$(1)$ Two isomers of butane are $n$-butane and isobutane. The relation of the number of $\sigma$ and $\pi$-bonds in them is .......... .
$(2)$ .......... is present in the structure of a heterocyclic compound.
$(3)$ Cyclohexene is a .......... type and furan is a .......... type of compound.
$(4)$ .......... type of $3-D$ model indicates the atomic volume but does not indicate the bond.

What is the molar mass of the third member of a homologous series if the molar mass of the first member is $46 \ g \ mol^{-1}$?