ISA Certified Automation Professional (CAP) program

The correct answer is A, "Maintainability is a front-end, design outcome; maintenance is related to ongoing system availability."

Maintainability is the probability that a device will be restored to an operating condition within a specified period when maintenance is done with prescribed resources and procedures. It can also refer to the inherent characteristic of a design or installation that determines the ease, economy, safety, and accuracy with which maintenance actions can be performed on it.

As such, maintainability should be addressed during the front-end engineering and design phase of a project, so these characteristics are built in to the process. This includes addressing items like accessibility for removing pumps, piping, and instruments; complete documentation and procedures; availability of spare parts; personnel training and qualification; and suitability for purpose.

Maintenance is what is performed on a system to ensure that it remains in good working condition, but also, if a failure should occur, maintenance is the mechanism used to return the system to the previous working condition.

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006.