Question

Mauna loa which means long mountain is the active volcano. Main Clause...?subordinate clause..?

Answer 1

main clause = is
subordinate clause =which

Answer 2

A subordinate clause depends on the main clause to have a proper meaning. So, along with the main clause, the subordinate clause will form a complex sentence. The subordinate clause does not make a complete sensible thought by itself.

The main clause will be - "is the active volcano".

The subordinate clause will be - "which means long mountain".

'Which' is used as the relative pronoun in the subordinate clause here.