By what fraction selling price must be multiplied to get the cost price, if the loss is 20% = ?

Answer: Option C

$$\begin{array}{rl}& \%\text{ Loss }=20\%\\ & 20\%=\frac{1}{5}\end{array}$$
It Cost price = 5 units
Loss = 1 units
Selling price = 5 - 1 = 4 units
Selling price = 4
we make it equal to Cost price that is (5)
So, we multiply Selling price by
$$\begin{array}{rl}& =\frac{5}{4}\\ & \text{Selling price}=4\times \frac{5}{4}\\ & =5\end{array}$$