(c) $q$
Since $p$ is a multiple of $q$, we have $p = kq$ for some natural number $k$. Thus $q$ divides $p$, making $q$ the highest common factor. So HCF$(p, q) = q$.
When one number is a multiple of the other, the smaller number ($q$) is always the HCF, because $q$ divides both itself and $p$ completely. The largest such divisor is $q$ itself.