Answer
(d) $125\,atoms\,s^{-1}$
Work Step by Step
Number of half-lives= $\frac{t}{t_{1/2}}=\frac{3\,h}{1.00\,h}=3$
As the sample undergoes 3 half-lives, the current rate decreases by half 3 times.
The new disintegration rate=$1000\,atoms/s\times(\frac{1}{2})^{3}=125\,atoms/s$
