Answer
$7.5×10^{-10}\;\rm kg.m/s$
Work Step by Step
We know that the impulse is given by
$$J=F\Delta t\tag 1$$
We also know that the force exerted on the pollen grains is constant (since the acceleration is constant) and is the only force exerted on them.
So that,
$$F=ma$$
Plugging into (1);
$$J=ma\Delta t $$
Plugging the known;
$$J=(1\times10^{-7}\times10^{-3})(2.5\times10^4)(0.3\times10^{-3})$$
$$J=\color{red}{\bf 7.5×10^{-10}}\;\rm kg.m/s$$
