If a certain sum becomes double in 3 years at certain rate of interest at C I Then in how many years...........

Explanation:

In C.I P increases like
P——-2P——–4P———8P——–16P
—-3yrs—–3yrs—–3yrs——-3yrs
total = 3+3+3+3=12 years

Hence, option 1 is the correct answer.