 Solution

For $x\in {\mathbb{R}}^d$, the euclidean projection onto a convex set $C$ isThe squared distance $z\mapsto \Vert x-z{\Vert }_2^2$ is a strictly convex function. Its restriction to the convex set $C$ can therefore have at most one minimizer by part (b). Closedness, together with finite dimensionality, supplies existence, although existence was not required to be proved.