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!

Explanation
You can add a reference line to any continuous axis in the view.
To add a reference line:
Drag Reference Line from the Analytics pane into the view. Tableau shows the possible destinations. The range of choices varies depending on the type of item and the current view.
In a simple case, the drop target area offers three options:

Explanation
The following is the table for the data types in Tableau along with their icons:

SUM is an aggregate function, not a table calculation!
A table calculation is a transformation you apply to the values in a visualization. Table calculations are a special type of calculated field that computes on the local data in Tableau. They are calculated based on what is currently in the visualization and do not consider any measures or dimensions that are filtered out of the visualization.
The most common Table calculations are:
Running Total
Percent Difference
Difference
Percent of Total
Rank
Percentile


These can be calculated using : Table(across), Cell, or Specific dimensions!

Explanation
For a manual sort, no matter how the data changes, the values will always stay in the sort order you kept stuff in.
From the official website: