In this sentence:
Its shape resembles that of a kangaroo’s hind foot.
What part of speech would that be?
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Here "that" stands for "the shape" (of a kangaroo's hind foot). Here the demonstratve adjective is used as a pronoun (without a following noun) and it has the sense of a determinative pronoun. English has no special words for determinatives, it uses as substitute the demonstratives or that which /what, the one who, people who, anyone who, he who, they who, whoever, whatever.
In that sentence, that would be a relative pronoun.