As the DU (degree of unsaturation for C5H10 is 1).
It can either be a ring or with one double bonded.
Since for cyclic compounds,
The isomers are:
Cyclopentane 
Methylcyclobutane 
1,2-dimethylcyclopropane .
						

							As the DU/DBE of C5H10 is 1.It can either be a ring or with one double bond.1. Cyclopentane2. Methylcyclobutane3. 1,2 Dimethylcyclopropane4. Ethylcyclopropane
						

							
Cyclopentane methylcyclobutane ..1,2 dimethyl cyclopropane 1,1 dimethyl cyclopropane ..ethyl cyclopropane
 
						

							Hi , it`
						

							4 isomers can be possible1) cyclopentane2) methylcyclobutane3) 1,2 dimethyl cycloprpane4) Ethylcyclopropane
						

							5 isomers can be possible1) cyclopentane2) methylcyclobutane3) 1,2 dimethyl cycloprpane4) Ethylcyclopropane5)1,1-Dimethylcyclopropane
						

							
the correct answer is 7 isomers.
 

						

							The answer came to be seven as the isomers who are optically active have two enantiomers and there are 2 cyclic structures which are optically active so answer is seven...
						

							
5 isomers are possible 
1- cyclopentane
2- methyl cyclobutane
3- ethyl cyclopropane
4- 1,1dimethylcyclopropane
5- 1,2dimethylcyclopropane