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 ages (A=23, N=3)

⇒S=23 X 3 =69

Later, if the age of 4th friend added, the average remains 23 (so A=23, N=4)

So, sum of 4 friends ages

⇒S=23 X 4 =92

Now,

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

= 92-69

= 23 yr.

Hence, Option C is correct.