The word partition refers to the act of dividing any space into two parts or more.
In the above sentences, the sentence which uses the word partition correctly is 2. Becky stood behind the partition that separated the audience from the people backstage and waited for her cue to enter.
In the above sentence, the words partition separates the two spaces, space of audience with that of space of people backstage.
Other sentences do not confine with the proper use of word partition because it relates the division of object and not spaces. Hence, sentence two is correct.
