Explanation
The following options are available when an image is Exported:

NOTE: When we Copy an image rather than exporting it, then the image is copied to the clipboard in the TIFF file format! However, it is not available when EXPORTING an image.

Explanation
After making changes to Data, we can save that new data source as a .tds file. To do so, go to data menu on top and then choose your current connected data source. Then next click on Add to Saved Data sources. This will save all calculated fields, changes to fields etc. It will be saved in My Tableau Repository -> Mydatasources. This will then also appear on Tableau Home Page under saved data sources like SampleSuperStore.
Note: Data source files do not contain the actual data but rather the information necessary to connect to the actual data as well as any modifications you've made on top of the actual data such as changing default properties, creating calculated fields, adding groups, and so on.
twb and .twbx are not the BEST solutions since the questions nowhere mentions that we need to store our workbooks as well.
twm is a bookmark which contains a single worksheet and are an easy way to quickly share your work.

Explanation
Unioning is the correct answer!
From the official documentation:

For example, suppose you have the following customer purchase information stored in three tables, separated by month. The table names are "May2016" , "June2016" and "July2016."

Explanation
This question is directly from the Official Tableau Desktop Specialist exam guide.
Since we are talking about the AVERAGE home values for the United States in 2016, the question is directly offering us a hint that the answer has something to do with aggregation and that too the values tell us that we're working with MEASURES.
Date part and Date values don't really make much sense given the question, and neither does geography.
Therefore, the answer naturally is "An aggregated measure".

Explanation
You can show trend lines in a visualization to highlight trends in your data.
To add trend lines to a view, both axes must contain a field that can be interpreted as a number. For example, you cannot add a trend line to a view that has the Product Category dimension, which contains strings, on the Columns shelf and the Profit measure on the Rows shelf.
However, you can add a trend line to a view of sales over time because both sales and time can be interpreted as numeric values.