How many structural isomers are possible for a molecule of ${{C}_{6}}{{H}_{12}}$ composition but having at least one carbocyclic ring?
(a)- Seven
(b)- Six
(c)- Five
(d)- Three

Hint: ${{C}_{6}}{{H}_{12}}$ means there are 6 carbon atoms present. Carbocyclic means that there is a ring present in the molecule. The rings can be of 3-ring, 4-ring, 5-ring, and 6-ring structures.

Complete answer:
So let us see the isomers one by one:
(a)- Cyclohexane
It is a ring structure of six carbon atoms as the name suggests. All the carbon has 2 hydrogens each. The structure is given below:


(b)- 1-Methyl cyclopentane
It is a ring structure of five carbon atoms and one methyl group is attached to the carbon atom of the ring. The structure is given below:


(c)- 1-Ethyl cyclobutane
It is a ring structure of four carbon atoms and one ethyl group is attached to the carbon atom of the ring. The structure is given below:


(d)- 1-n-propyl cyclopropane
It is a ring structure of three carbon atoms and at one of the carbon, a straight three carbon chain is present. The structure is given below:


(e)- 1-isopropyl cyclopropane
 It is a ring structure of three carbon atoms and at one of the carbon a straight chain of three carbon is present and the carbon of the ring is attached to the second carbon of the chain. The structure is given below:


(f)- 1,2-Dimethyl cyclobutane
It is a ring structure of four carbon atoms and 2 methyl groups are present at the adjacent carbon atom of the ring. The structure is given below:


(g)- 1,3-Dimethyl cyclobutane
It is a ring structure of four carbon atoms and 2 methyl groups are present at the opposite carbon atom of the ring. The structure is given below:


So, the correct answer is “Option A”.

Note: There are more isomers of hexane like 1,1-Dimethyl cyclobutane, 1-ethyl-2-methyl cyclopropane, 1-ethyl-1-methyl cyclopropane, 1,2,3-trimethyl cyclopropane, etc. There are more isomers of hexane like 1,1-Dimethyl cyclobutane, 1-ethyl-2-methyl cyclopropane, 1-ethyl-1-methyl cyclopropane, 1,2,3-trimethyl cyclopropane, etc. But these are not structural isomers having at least one carbocyclic ring . Thus they are not counted here.