A forest is a collection of trees. here we are given a forest with $n$ vertices and $k$ components. a component is itself a tree.
Since there are $k$ components means that every component has a root (every tree has one), therefore we have $k$ roots.
Introduction of each new vertex to the forest introduces a single edge to a forest. so for remaining $n-k$ vertices when introduced, to make up to $n$ vertices, contributes to $n-k$ edges.
Hence, ans $=$ option (C) $= (n-k)$