c
Given \(t_{1/2}\,=\,15\,minutes\)

Total time \((T)\,=\,1\,hr\,=\,60\,min.\)

From \(T\,=\,n\times t_{1/2}\)

\(n\,=\,\frac {60}{15}\,=\,4\)

Now from the formula \(\frac{N}{{{N_0}}}\, = \,{\left( {\frac{1}{2}} \right)^n}\)

                              \( = \,{\left( {\frac{1}{2}} \right)^4}\, = \,\frac{1}{{16}}\)

Where \(N_0\,=\) initial amount

\(N\,=\) amount left after time \(t\)

hence the amount of substance left after \(1\,hour\) will be \(\frac {1}{16}\)