The half life of a radioactive substance is 20 s. The decay constant is

$0.0346\, s^{-1}$

The correct answer is Option (1) → $0.0346\, s^{-1}$

$\text{Given: Half-life } T_{1/2} = 20~\text{s}$

$\text{Relation between half-life and decay constant: } T_{1/2} = \frac{\ln 2}{\lambda}$

$\lambda = \frac{\ln 2}{T_{1/2}} = \frac{0.693}{20}$

$\lambda \approx 0.03465~\text{s}^{-1}$

$\text{Answer: } \lambda \approx 0.0347~\text{s}^{-1}$