A set is a mathematical model for a collection of items. The correct option is C.
<h3>What is a set?</h3>
A set is a mathematical model for a collection of items; it comprises elements or members, which may be any mathematical object: numbers, symbols, points in space, lines, other geometrical structures, variables, or even other sets.
Given the s={(a, b), (a, c), (b, d), (d, c)} and t={(b, b), (c, a), (c, d), (a, d)} Therefore, the value of t ∘ s = { (a, c), (b, d), (c, b), (c, c) }.
Hence, the correct option is C.
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