Molecules having whole number for degree of unsaturation can exist in free state as stable compounds.
Degree of unsaturation $=\frac{\Sigma n(v-2)}{2}+1$
where, $n=$ number of atoms of a particular type $v=$ valency of the atom.
(a) For $\mathrm{C}_{7} \mathrm{H}_{9} \mathrm{O}$,
$D U=\frac{7(4-2)+9(1-2)+1(2-2)}{2}+1=3 \cdot 5$
(b) For $\mathrm{C}_{8} \mathrm{H}_{12} \mathrm{O}$
$$
\mathrm{DU}=\frac{8(4-2)+9(1-2)+1(2-2)}{2}+1=3 \cdot 0
$$
(c) For $\mathrm{C}_{6} \mathrm{H}_{11} \mathrm{O}$
$$
\mathrm{DU}=\frac{6(4-2)+11(1-2)+1(2-2)}{2}+1=1 \cdot 5
$$
(d) For $\mathrm{C}_{10} \mathrm{H}_{17} \mathrm{O}_{2}$
$D U=\frac{10(4-2)+17(1-2)+2(2-2)}{2}+1=2 \cdot 5$
$\therefore \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}$ will exist in free state or a stable compound.