正解:A
According to the CISM Review Manual, engaging stakeholders is the most important step when developing an information security strategy, as it helps to ensure that the strategy is aligned with the business objectives, expectations, and requirements of the stakeholders. Engaging stakeholders also helps to gain their support and commitment for the implementation and maintenance of the strategy. Assigning data ownership, determining information types, and classifying information assets are possible subsequent steps, but not the most important one.
Reference = CISM Review Manual, 27th Edition, Chapter 2, Section 2.1.1, page 731.