|Which is correct?
The development team, in collaboration with end-users, _____ creating a prototype.
Answer: a (is)
Team is the subject of the sentence. It’s singular, so it takes a singular verb, is. In conjunction with end-users is a prepositional phrase that modifies team.
If we change the sentence to say “The development team and end-users __ creating a prototype,” however, the correct answer would be are.