If $N_0$ is the original mass of the substance of half life $t_{\frac{1}{2}}=4$ years, then the amount of substance left after 12 years is:

$N_0 / 8$

The correct answer is Option (3) → $N_0 / 8$

The exponential Decay law is,

$N=N_0\left(\frac{1}{2}\right)^{\frac{t}{t_{1/2}}}$

$⇒N=N_0\left(\frac{1}{2}\right)^{\frac{12}{4}}$

$⇒N=N_0×\frac{1}{8}=\frac{N_0}{8}$