(a) One double bond. $\left( \mathrm{C}_3\mathrm{H}_6\right.$ Lewis structure contains three carbon atoms bonding with six hydrogen atoms, all these are bonded with the 7 single bonds and 1 double bond).(b) $\mathrm{C}_4\mathrm{H}_6$ is an alkyne,The alkynes comprise a series of carbonand hydrogen-based compounds that contain at least one triple bond. The group of compounds is a homologous series with the general molecular formula of $\mathrm{C}_n\mathrm{H}_{2n-}$ ${}_2$, where $n$ equals to any integer greater than one. Thus, the first member of this homologous series is ethyne $\left( \mathrm{C}_2\mathrm{H}_2\right) . \mathrm{H}-$ $\mathrm{C} \equiv \mathrm{C}-\mathrm{H}$(c) $\mathrm{C}_6\mathrm{H}_6$ (Benzene) will form the ringThe structure has a six-carbon ring which is represented by a hexagon and it includes 3-alternate double bonds.(d) $\mathrm{C}_3\mathrm{H}_8$ is an alkane as it has each carbon bonded to four other atoms through single covalent bonds and follow general formula $\mathrm{C}_n\mathrm{H}_{2n+2}$