Homologues have different

The enolic form of acetone contains:

An organic compound $C_3H_6O$ does not give a precipitate with $2,4-$Dinitrophenylhydrazine reagent and does not react with metallic sodium. The compound is:

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.

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

Which of the following statements is correct?