A tool for monitoring how lean principles are working in an organization:  A. Poka-yoke  B. Andon  C. Gemba  D. Lean Audit

The correct answer is D. Lean Audit. A Lean Audit is a tool used to monitor and evaluate how effectively lean principles are being applied within an organization. It involves a systematic review of operations, processes, and practices to identify areas of waste or inefficiency that can be improved.

Here's a brief explanation of each option:

A. Poka-yoke: This is a Japanese term that means "mistake-proofing". It is a mechanism in lean manufacturing process that helps an equipment operator avoid mistakes. Its purpose is to eliminate product defects by preventing, correcting, or drawing attention to human errors as they occur.

B. Andon: This is a system to notify management, maintenance, and other workers of a quality or process problem. The alert can be activated manually by a worker using a pullcord or button or may be activated automatically by the production equipment itself.

C. Gemba: This is a Japanese term meaning "the real place." In business, gemba refers to the place where value is created; in manufacturing the gemba is the factory floor.

D. Lean Audit: This is a tool for monitoring how lean principles are working in an organization. It helps to identify whether the principles of lean are being followed and where improvements can be made.