The correct option is (C) 10.
Explanation:
In t = 40 days, substance decays to (1 / 16) of original i.e. fraction left is (1 / 16).
If half life is T(1/2) then fraction left = (N / N0) = (1 / 2)[t / {T(1/2)}]
(1 / 16) = (1 / 2)[(40) / {T(1/2)}]
(1 / 2)4 = (1 / 2)[(40) / {T(1/2)}]
4 = [(40) / T(1/2)]
T(1/2) = (40 / 4) = 10 days.