c
Suppose n resistors are used for the required job. Suppose equivalent resistance of the combination is \(R\)' and according to energy conservation it's current rating is \(i'.\)

Energy consumed by the combination \(= n×\) (Energy consumed by each resistance)

\(⇒\) \(i{'^2}R' = n \times {i^2}R\)  \(⇒\)  \(n = {\left( {\frac{{i'}}{i}} \right)^2} \times \left( {\frac{{R'}}{R}} \right) = {\left( {\frac{4}{1}} \right)^2} \times \left( {\frac{5}{{10}}} \right) = 8\)