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 workers age (A=23, N=4)

⇒S=23 X 4 =92

Later, the age of 5th worker is added, the average remains 23. ( so A=23, N=5)

So, sum of 5 workers age

⇒S=23 X 5 =115

Now,

The age of 5th worker =(sum of 7 observations)-(Sum of 6 observations)

  =115-92

 =23 yr.

Hence, Option C is correct.