$A$ hydrocarbon $C_{5}H_{10}$ does not react with chlorine in the dark but gives a single monochloro compound $C_{5}H_{9}Cl$ in bright sunlight. Identify the hydrocarbon.

(N/A) hydrocarbon with the molecular formula $C_{5}H_{10}$ belongs to the group with a general molecular formula $C_{n}H_{2n}$.
Therefore,it may either be an alkene or a cycloalkane.
Since the hydrocarbon does not react with chlorine in the dark,it cannot be an alkene (alkenes undergo addition reactions with chlorine even in the dark).
Thus,it must be a cycloalkane.
Further,the hydrocarbon gives a single monochloro compound,$C_{5}H_{9}Cl$,by reacting with chlorine in bright sunlight.
Since a single monochloro compound is formed,all hydrogen atoms in the hydrocarbon must be equivalent.
In cyclopentane,all $10$ hydrogen atoms are equivalent due to the symmetry of the ring.
Hence,the hydrocarbon is cyclopentane.