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 12 players age (A=18, N=12)

⇒S=18 X 12 =216

Later, If the age of the coach is not consider, the average decrases by 1 yr.  (so A=17, N=11)

So, sum of 11 players age

⇒S=17 X 11 =187

Now,

The age of the coach =(sum of 12 players age)-(Sum of 11 players age)

=216-187

=29 yr.

Hence, Option A is correct.