Data blending is performed on a sheet-by-sheet basis and is established when a field from a second data source is used in the view. To create a blend in a workbook already connected to at least two data sources, bring a field from one data source to the sheet-it becomes the primary data source.
Switch to the other data source and use a field on the same sheet-it becomes a secondary data source. An orange linking icon will appear in the data pane, indicating which field(s) are being used to blend the data sources.

According to the official Tableau Documentation:

To read more about Data Blending, click on THIS link.

A bullet graph is a variation of a bar graph developed to replace dashboard gauges and meters. A bullet graph is useful for comparing the performance of a primary measure to one or more other measures. Below is a single bullet graph showing how actual sales compared to estimated sales.
We can create a Bullet graph with just 2 measures! This method requires the LEAST number of fields possible to create this type of chart.
The best way to tackle such questions in the exam is to click the "SHOW ME" button on top right, and hover over the chart we want to create.
In our case, it is a Bullet graph.


Therefore, we need 2 measures at least to create this chart, and 0 or more dimensions.

Explanation
Data blending is performed on a sheet-by-sheet basis and is established when a field from a second data source is used in the view. To create a blend in a workbook already connected to at least two data sources, bring a field from one data source to the sheet-it becomes the primary data source.
Switch to the other data source and use a field on the same sheet-it becomes a secondary data source. An orange linking icon will appear in the data pane, indicating which field(s) are being used to blend the data sources.

According to the official Tableau Documentation:

To read more about Data Blending, click on THIS link.

This is one of the most common questions on the Tableau Desktop Specialist Exam.
Wrong options -
1) Don't use backgrounds - This is not a solution. What if we want to use backgrounds? We can't just stop using backgrounds to solve this problem.
2) Decrease the font size - Do you think using a smaller font will make the text more readable? No right? Hence, this is wrong too.
All other options are ways recommended to make your text more readable!

Presently, there is no official way to connect your data in Firebase directly with Tableau Desktop.
A workaround however can be to export your Firebase data into Google BigQuery, and then connect it to Tableau Desktop.
But then again, it is a workaround and not an official out-of-the-box solution.
The following are the available Data sources available as of now:
1) Server

2) File