# P vs NP characterization confusion

I know that $$P \subseteq NP$$, but for a problem in $$P$$, e.g. MST in a graph, is it a correct statement to say that:

The MST problem belongs in NP-Class.

(I mean, i think it is correct, but could someone classify that as wrong because he would expect P instead of NP?)

The statement is correct for exactly the reason you started with: $$\mathsf P$$ is a subset of $$\mathsf{NP}$$ which means that every problem in $$\mathsf P$$ is also in $$\mathsf{NP}$$. You can also go through the definitions and find that a deterministic TM running in polynomial time is merely a special kind of nondeterministic TM running in polynomial time and take it from there.
However, while being true, the statement "MST is in $$\mathsf{NP}$$" holds less value than the statement "MST is in $$\mathsf P$$" as complexity theory is, at large, concerned with lower bounds. A decent analogy in my opinion would be that the statement $$\pi \geq 3$$ is stronger than the statement $$\pi \geq 0$$ eventhough both are true.