According to the question

We use the formula,

Value of new observations= a - nb 

[here '-' sign (a-nb) has been taken as average decreases in this case]

Here, n=30, a=25 yr, b= $\frac{3}{12}$ yr

Age of keerthi = a - nb

= 25-30×$\frac{3}{12}$

= 25-$\frac{15}{12}$

= 25-7.5= 17.5 yr

Hence, Option A is correct.