theirs


Concise Oxford English Dictionary © 2008 Oxford University Press:
theirs/ðeəz/
possessive pronoun used to refer to something belonging to or associated with two or more people or things previously mentioned.
– origin ME: from their + -s3.
usage: There is no need for an apostrophe: the spelling is theirs not their's.
'theirs' also found in these Oxford entries:

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