A. Custom Fields
D. Workflow Rules
E. Field Tracking

The three types of customization that can be done on Activities (tasks and events) are:

A. Custom Fields: You can add custom fields to tasks and events to capture additional data that is not included in standard fields.

D. Workflow Rules: You can create workflow rules that are triggered by activities to automate processes, such as sending email alerts, creating tasks, or updating records.

E. Field Tracking: You can track changes to standard and custom fields on tasks and events to monitor updates and changes made by users.

Assignment Rules (B) and Validation Rules (C) are not applicable to Activities. Assignment Rules are used to automatically assign records to users or queues based on specific criteria, and Validation Rules are used to ensure that data entered by users meet specific criteria before it is saved.