The correct answer is A) Condition-based maintenance. Condition-based maintenance is a proactive approach where equipment is monitored and maintenance is performed based on the actual condition of the equipment, rather than on a fixed schedule or after a failure has occurred. This allows for more efficient and cost-effective maintenance, as it prevents unnecessary maintenance and addresses issues before they lead to breakdowns or other problems. The other options represent more reactive approaches to maintenance. Unscheduled maintenance (B) is performed in response to unexpected issues, while failure-based maintenance (C) and corrective maintenance (D) are carried out after a failure has already occurred, which can lead to more costly and disruptive consequences.