To allow users to create repeating tasks, a Salesforce Administrator must perform two key actions. First, the feature must be enabled at the organizational level. This is done by navigating to 'Activity Settings' in Setup and selecting 'Enable Creation of Recurring Tasks'. Second, for users to access this functionality on the user interface, the controlling field must be made visible on their Task page layouts. This field, typically a checkbox labeled 'Repeat', allows users to define the recurrence pattern for a task series.

B. Disable shared Activities: Shared Activities (relating a task to multiple contacts) is a separate feature. Disabling it is not a prerequisite for enabling or using repeating tasks.

D. Turn on Task Notifications service: Task notifications are related to user alerts and reminders for upcoming activities, not the fundamental ability to create a recurring series of task records.

1. Salesforce Help - Enable Creation of Recurring Tasks:

Source: Official Salesforce Documentation.

Details: This document explicitly outlines the two-step process.

Step 1: "From Setup, enter Activity Settings in the Quick Find box, then select Activity Settings. Select Enable Creation of Recurring Tasks." This directly supports option A.

Step 2: "Make sure that the Repeat checkbox is on your task page layouts." This directly supports option C, as this checkbox is the field that allows users to "Create [a] Recurring series of Tasks".

2. Salesforce Help - Considerations for Using Recurring Tasks:

Source: Official Salesforce Documentation.

Details: This document lists limitations of recurring tasks but does not mention any requirement to disable Shared Activities. It notes that a recurring task cannot be assigned to multiple users, but this is a functional limitation, not a setup dependency, confirming option B is incorrect.

3. Salesforce Help - Set Reminders in Lightning Experience:

Source: Official Salesforce Documentation.

Details: This documentation describes how activity reminders and notifications work. The functionality is entirely separate from the creation of recurring records, confirming that option D is unrelated to the user's problem.