NEET Physics Nuclei Questions Solved

The half-life of ${}^{215}\mathrm{At}$ is 100 $\mathrm{\mu s}$. The time taken for the radioactivity of a sample of ${}^{215}\mathrm{At}$ to decay to 1/16th of its initial value is
(a) 400 $\mathrm{\mu s}$               (b) 6.3 $\mathrm{\mu s}$
(c) 40 $\mathrm{\mu s}$                 (d) 300 $\mathrm{\mu s}$