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 3 friends age (A=32, N=3)

⇒S=32 X 3 =96

Later, the age of 4th friend added, the average come out to be 31 yr (so A=31, N=4)

So, sum of 4 friends age 

⇒S=31 X 4 =124

Now,

Age of the 4th friend =(sum of 3 friends age)-(Sum of 4 friends age)

=124-96

=28 yr.

Hence, Option B is correct.