A real estate broker is subject to disciplinary action from the Department of Real Estate (DRE) if they:

A is incorrect because while extracting undisclosed secret profits is a serious violation of fiduciary duty, it's primarily a breach between the broker and their client rather than a direct violation that would automatically trigger DRE disciplinary action in this context.

B is incorrect because commingling trust funds is a violation, but it's not the correct answer in this specific question. While it's a serious offense that can lead to disciplinary action, it's not the best answer when compared to C.

D is incorrect because not all the listed options would necessarily result in DRE disciplinary action. While commingling funds (B) would likely lead to action, and failing to supervise (C) definitely would, extracting secret profits (A) might be handled through civil remedies rather than automatic DRE discipline.