The correct answer is C) Every four hours. According to the regulations for periodically unmanned engine rooms, the duty engineer is required to perform machinery space rounds every four hours when the engine room is unattended. This ensures that the engineer regularly checks the status of the machinery and equipment, and can quickly identify and address any potential issues that may arise during the unattended periods. The other options are incorrect because A) performing rounds every hour would be overly frequent, B) every two hours would not provide sufficient monitoring, and D) every twelve hours would be too infrequent and could allow problems to go unnoticed for too long.