Here we can use the formula

$$\begin{gathered} \text{Average (A)=}\frac{\mathrm{Sum} \mathrm{of} \mathrm{observations} \left(\mathrm{S}\right)}{\mathrm{No}. \mathrm{of} \mathrm{observations} \left(\mathrm{N}\right)} \\ \text{⇒ S=A×N} \end{gathered}$$

According to the question,

Sum of 4 big book shelf and a small book shelf (A=120, N=5)

⇒S=120 X 5 =600

Later, the small book shelf are eliminated, the average deacreases by 50. (so A=70, N=4)

So, sum of 4 big book shelf

⇒S=70 X 4 =280

Now,

The number of books in small book shelf =(sum of 4 big book shelf and a small boook shelf)-(Sum of 4 big book shelf)

=600-280

=320

Hence, Option C is correct.