Answer
${\bf 2.0}\%$
Work Step by Step
We need to find the probability of decay over a short interval $\Delta t = 0.50 \, \text{ns}$, compared to the total lifetime $\tau = 25 \, \text{ns}$.
We know that the probability of decay during the interval $\Delta t$ can be expressed as:
$$
\text{Prob(decay in } \Delta t) = r \Delta t
$$
where the decay rate $r$ is given by $r = \dfrac{1}{\tau}$, so
$$
\text{Prob(decay in } \Delta t) = \dfrac{\Delta t}{\tau}
$$
Plug the known;
$$
\text{Prob(decay in } \Delta t) = \frac{0.5}{ 25}= \bf 0.020 =\color{red}{\bf 2.0}\%
$$
