Which term refers to the buddhist belief of union with the universe and release from the cycle of rebirth?

Nirvana. It is the goal in following the Noble Eight-fold Path.

Nirvana

- in Buddhist belief, union with the universe and release from the cycle of rebirth

This is the term for the Buddhist belief in a salvation. This salvation includes inner peace and becoming one with the universe. Buddhist must go through many cycles of reincarnation and when they achieve Nirvana they are released from it. To achieve nirvana it takes lots of meditation, being a kind and good person and you must go through many lives through reincarnation.

the ansseer is going to be c i think