Maximum enol content is in

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

On mixing tetraethyl lead to gasoline available at petrol pumps,

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.

Which of the following statements is $NOT$ true about a homologous series?

The enolic form of acetone contains: