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

What is obtained when $4$-methylbenzenesulfonic acid reacts with sodium acetate?

The least acidic compound among the following 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.

The number of secondary carbon atoms present in the following compounds are respectively:

The enolic form of acetone contains: