In a sample of radioactive substance what fraction of initial no . of nuclei will remain undecayed after time $\;t=\large\frac{T}{2}\;$ , where T = half - life of substance

Question

$(a)\;\sqrt{2}\qquad(b)\;\large\frac{1}{ \sqrt{2}}\qquad(c)\;\large\frac{1}{3 \sqrt{2}}\qquad(d)\;None$