Answer:
theirs
Step-by-step explanation:
well when you say The yellow and green backpacks are them? does that make sense? does themselves make sense? it wont be "these"(in quotation cuz I am talking about the word not the other options previous)because you are talking about a possessive noun and pronouns are a word that you use to refer to someone or something when you do not need to use a noun, often because the person or thing has been mentioned earlier, for example it, she, him, etc. "these" wouldn't be it even though it makes sense in the sentence, it is talking about who they belong to, not what they are