"He starts by sprinting 100 yards", so the first term is a_1 = 100
The common difference is d = 5 because he's adding 5 yards per day. The "21 days" info isn't going to be used at all
So because a_1 = 100 and d = 5, this means
a_n = a_1 + d*(n-1)
a_n = 100 + 5*(n-1)
a_n = 100 + (n-1)*5
Answer is choice D