A first order reaction is 50% completed in 1.26 × 10¹⁴ s. How much time would it take for 100% completion?

Given, t_1/2=1.2610¹⁴ we know the reaction is a first order

t_1/2=.693/k ……..for first-order reaction

using above given value we can find k

For 100% completion

At 100% completion R=0 so

Thus Option (iv) is correct.