Final answer:
The pronoun case used in the sentence is possessive.
Step-by-step explanation:
The pronoun case used in the sentence 'copies of the memo were left on their desks by Brian' is the possessive case. The pronoun 'their' is indicating ownership, showing that the desks belong to someone. The possessive case pronouns include my/mine, our/ours, your/yours, his/her/hers/its, their/theirs, and whose.
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