The interval is compact by the Heine-Borel theorem, so its square is compact by part (a). The half-open interval is not compact because it is not closed in the real line.
Each of the other two products has a continuous surjective coordinate projection onto . If either product were compact, that image would be compact, a contradiction. Compactness is preserved by homeomorphisms, so neither product is homeomorphic to the closed square.
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