Show that finding a minimum-weight subgraph that includes all marked nodes is NP-hard We've been given a weighted graph with marked nodes. We want to make a minimum-weight subtree from this graph that contains all marked nodes.

I want to show that this problem is NP-hard. Is there any idea to which problem is the best for reduction to this problem?
This is the problem of finding a Steiner tree in the graph. It's one of the NP-complete problems in Karp's original paper; the reduction there is from Exact cover by 3-sets.