As the Degree of Unsaturation (DU) for C5H10 is 1. It can either be a ring or with one double bonded.

Since for cyclic compounds, the isomers are:
-
cyclopentane (CH2)5
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methyl butane [CH(CH3)CH2CH2CH2]
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1,1-dimethyl propane [CH(CH3)CH(CH3)CH2]
-
1,2-dimethyl propane
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1,3-dimethyl propane
-
1,4-dimethyl propane
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ethyl propane [CH(C2H5)CH2CH2]