Very important question for the exam and appears quite a lot too.
The correct answer is - By dragging another dimension to Color on the Marks card.

This is what the question says we have already created. Now to convert this into a Stacked bar chart, we will drop another dimension on Color in the Marks card.

The rest won't create stacked bar charts, and hence are incorrect choices. The best way to answer such questions on the real exam is to quickly do what the options say and see if they satisfy the requirements in the question.

Tables that you drag to the logical layer of the Data Source page canvas must be related to each other. When you drag additional tables to the logical layer canvas, Tableau automatically attempts to create the relationship based on existing key constraints and matching fields to define the relationship. If it can't determine the matching fields, you will need to select them.
If no constraints are detected, a Many-to-many relationship is created and referential integrity is set to Some records match. These default settings are a safe choice and provide the most a lot of flexibility for your data source.

If you've created a data connection that you might want to use with other workbooks or share with colleagues, you can export (save) the data source to a file. You might want to do this also if you've added joined tables, default properties, or custom fields-such as groups, sets, calculated fields, and binned fields-to the Data pane.

Visualisations and Data extracts are NOT saved in a .tds file!

A URL action is a hyperlink that points to a web page, file, or other web-based resource outside of Tableau. You can use URL actions to create an email or link to additional information about your data. To customize links based on your data, you can automatically enter field values as parameters in URLs.

According to the official Tableau documentation, there are 5 types of trend lines which we can work with in Tableau :
1) Linear Trend Line
2) Logarithmic Trend Line
3) Exponential Trend Line
4) Polynomial Trend Line
5) Power Model
Hence, the correct answer is BINOMIAL trend line which is not present in Tableau.
See the following image:

For more information, refer to: https://help.tableau.com/current/pro/desktop/en-us/trendlines_add.htm