They wanted to go on an adventure together. they decided to visit the south pole
a. compound verb
b. compound object
c. compound subject

This is Compound Verb ( A compound verb is a verb  that uses multiple words to form a single verb ) there are four type of compound verb :-
prepositional  :- when a preposition combine with a verb  to add detail in it.
                               ( like :- believe in , care for )
Phrasal   :-  when a verb combine with a prepositional adverb .
                           (like :-  tear off , run out)
Auxiliary :-    when a verb combine with helping verb . and often used when talking in different tense . ( like :- was walking , will meet ) 
Combine single word :-  when single verb made from multiple words .sometimes they are joined sometimes differently. 
                    ( like :- babysit   , water proof , stir fry )