This is one of the story guidelines recommended by SAP Analytics Cloud. According to the course material1, "When designing your stories, keep them as simple as possible. In general, it is easier for users to consume story content if it is present in smaller bits rather than have a few large stories with many pages to navigate." The other options are not mentioned as guidelines in the course material.
1: Introducing SAP Analytics Cloud Story Design | SAP Training 2

The sort options that are available in stories are ascending and descending order, and multiple dimensions together. Ascending and descending order allow you to sort data by one or more dimensions or measures in ascending or descending order. Multiple dimensions together allow you to sort data by more than one dimension in a hierarchical order. Dimensions and measures together, and multiple measures together are not valid sort options in stories. Verified [SAP Analytics Cloud - Sorting]

The Digital Boardroom is a feature that allows you to create immersive and interactive presentations based on stories. You can use the Digital Boardroom to present your data and insights on multiple screens connected to a single device. The Digital Boardroom supports up to three screens that can display different views of the same story or different stories. Therefore, the correct answer is 3, as it is the maximum number of screens you can connect to the Digital Boardroom. Verified Reference:

When a model includes a prompt, the prompt is presented at runtime before data can be displayed in the story. The prompt allows users to select the values they want to filter the data by. The prompt is not related to the predefined widgets, the rendering of widgets, or the navigation to widgets. Verified [SAP Analytics Cloud - Prompting]

When you are adding a calculation to a table, you can choose from two options: Single or Multiple. The option determines how many new columns are added to the table based on the calculation. The options are:
Single: Adds one new column to the table with the calculation result for the selected dimension member. For example, if you select Soft Drinks and choose Single, you will get one new column with the calculation result for Soft Drinks only.
Multiple: Adds multiple new columns to the table with the calculation result for each dimension member. For example, if you select Soft Drinks and choose Multiple, you will get three new columns with the calculation result for Soft Drinks, Juice, and Alcohol.
Therefore, the correct answer is Selected dimension member, as it is the option that the new column is added to when you choose Single. Verified Reference:
: Create Calculations in Tables