The half life of $\;^{215}At\;$ is $\;200 \mu s\;$ . The time taken for radioactivity of a sample of $\;^{215}At\;$ to decay to $\;{\large\frac{1}{4}}^{th}\;$ of its initial value is

Question

$(a)\;100\;\mu s\qquad(b)\;300\;\mu s\qquad(c)\;200\;\mu s\qquad(d)\;400\;\mu s$