The length of each board is expressed as 15+h where h is tiny.
The length of two boards is 2(15+h)=30+k where k is the discrepancy in the total length.
So 30+2h=30+k making 2h=k. We are told k=0.06 in, so h=0.06/2=0.03 in.
This means the length of each board must be within 0.03 in of 15 in, answer choice d.