Past exam of the probability, statistics and options I course of the University of Oxford / 2007 / R / 1 / x

True. Each connected component of a forest is a tree. If the component sizes are $n_1,\dots ,n_k$, their edge counts are $n_1-1,\dots ,n_k-1$, including isolated vertices as size-one components. Summing gives $m=n-k$, hence