Full Detailed In-Depth Explanation:

Actions in Oracle Global Human Resources Cloud allow tracking and processing of employment changes, linked to Action Types for categorization.

Option A: While actions are accessible via Smart Navigator and security controls visibility, this statement is not the most direct answer to the question’s focus on creating and associating actions.

Option B: Incorrect. Multiple actions can be associated with a single Action Type (e.g., Voluntary and Involuntary under Termination).

Option C: Incorrect. An action reason is optional, not mandatory, depending on configuration and business rules.

Option D: Correct. Users can create custom (user-defined) actions (e.g., "Special Project Assignment") and link them to predefined Action Types (e.g., Assignment Change), enabling tailored tracking of changes.

The correct answer isD, aligning with the flexibility described in "Implementing Global Human Resources" for action customization.