Answer
(d) has its activity reduced by half.
Work Step by Step
At first, activity= $\lambda N$.
After one half-life, the concentration or the number of nuclei in the sample becomes half.
In that case, the number of undecayed nuclei $N'=\frac{N}{2}$ and
the new activity=$\lambda \times\frac{N}{2}=\frac{1}{2}\lambda N$
The activity of the sample is reduced by half after one half-life.
