Prashant's grandfather was 9 times older than him 16 years ago. His grandfather's age will be 3 times of his age 8 years from now. Eight years ago, the ratio of Prashant's age to that of his grandfather was:

Let Prashant's age 16 years ago = P, so Grandfather = 9P. Currently: Prashant = P+16, Grandfather = 9P+16. In 8 years: Prashant = P+24, Grandfather = 9P+24. Given: 9P+24 = 3(P+24), so 9P+24 = 3P+72, giving 6P=48, P=8. Eight years ago means Prashant was (P+16-8)=P+8=16, Grandfather was (9P+16-8)=9P+8=80. Ratio = 16:80 = 1:5. Option D is correct.