Исходное предложение: He bought a cake yesterday.
Тип предложения: Активный залог.
Объяснение: Субъект (He) выполнил действие (bought) над объектом (a cake) во времени Past Simple.
Пассивный залог:
Результат: A cake was bought by him yesterday.
Ответ: A cake was bought by him yesterday.