When exercised, an option written on which of the following items must be settled in...

Step by Step Explanation:

Equity Index Options: These are cash-settled because the underlying asset is not a physical security but a theoretical value representing the index.

Incorrect Options:

Preferred Stock, Master Limited Partnerships, and ETFs: These involve physical delivery of the underlying asset upon exercise.

Options Clearing Corporation (OCC) Guidelines: OCC Cash-Settled Options.