For a custom object to support Tasks and Events, the "Allow Activities" setting must be enabled in the object's properties. When a Platform Administrator creates a custom object, this checkbox is often left unchecked by default35. Enabling this feature allows the "Related To" (WhatId) field on a Task or Event to be linked to records of that custom object type36. While adding the related list to the page layout (Option C) is a necessary step for visibility, it will not work if the underlying feature isn't enabled first37. Sharing rules (Option A) and profile permissions (Option D) manage access to existing records but do not control whether the object is technically capable of having activities associated with it.