The reaction conditions leading to the best yields of $C_2H_5Cl$ are

Question

$\begin{array}{1 1}(a)\;C_2H_6(excess)+Cl_2\quad\underrightarrow {\text{UV light}}\\(b)\;C_2H_6+Cl_2\quad\underrightarrow {\text{dark}}\\(c)\;C_2H_6+Cl_2(excess)\quad\underrightarrow {\text{UV light}}\\(d)\;C_2H_6+Cl_2\quad\underrightarrow {\text{UV light}}\end{array}$