4th Grade Terranova Practice Test 2026 – Your All-in-One Guide to Mastering Exam Success!

The word needed shows who owns the books. In this sentence you want to say the books belong to them, so you use a possessive determiner before the noun. That makes the sentence flow naturally: They forgot their books at home.

There is a location word, there, which would be used to talk about where something is, not who owns it. They're is a contraction meaning they are, which doesn’t fit after forgot. Theirs would work only if you were replacing the noun phrase (The books are theirs), but not in this sentence as written. So the best choice is their.