Let CP $ = 100\%,\;$ MRP $ = 120\%$
Given that , $120\% \implies \text₹180$
then $100\% \implies x$
therefore, $x = \text₹150$
Now, the cost of the paper used in a single copy of the magazine $ = 25\%\;\text{of}\;x = \dfrac{25}{100} \times 150 = \text₹ 37.50.$
So, the correct answer is $(B).$