= Solution
A <functor> $U:\mathcal C\to\mathbf{Set}$ is a <representable functor> if some object $R$ admits a <natural isomorphism> $\mathcal C(R,-)\cong U$. Here the representable is covariant.
For the identity <functor> on the <Category of sets>, choose the singleton $1=\{*\}$. The evaluation maps
$$
\mathbf{Set}(1,X)\longrightarrow X,\qquad f\longmapsto f(*)
$$
are <bijections> with inverse $x\mapsto(*\mapsto x)$. For $u:X\to Y$, evaluation of $uf$ is $u(f(*))$, proving naturality. Hence \b[the identity functor on sets is represented by a singleton].
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