Final answer:
The Activity Log is the always read-only section on a CRM lead detail page because it is a historical record of interactions and should not be altered.
Step-by-step explanation:
The section that is always read-only on the lead detail page in many CRM systems, such as Salesforce, is the Activity Log. This is because the Activity Log is meant to provide a historical record of all interactions with the lead and needs to remain unaltered to ensure integrity of the data. Therefore, options A (Company information), B (Lead Information), C (Opportunity Information), and D (SFDC Custom Fields) are typically editable to some extent by users with the appropriate permissions. The Activity Log is valuable for sales and marketing teams to track progress and engagement with potential customers.