Which feature must be activated to allow you to use a tabular report in a table component?

Answer options:

A.Conditional Highlighting
B.Chart must be added to the source report
C.Custom Summary Formula
D.Rows to display must be set

Correct Answer: D 
To use a tabular report as the source report, Rows to Display must be set for that report. When you create a dashboard component to display your tabular report, you can use the dashboard component editor to override the settings you chose in Dashboard Settings. 
Option A is incorrect because conditional highlighting is used to highlight field values on summary or matrix reports based on ranges and colors you specify.
Option B is incorrect because adding a chart to the source report gives the users a visual way to understand the data in your report. 
Option C is incorrect because the custom summary formula is a great way to calculate additional totals based on the numerical values in your report. 
Reference: 

https://help.salesforce.com/articleView?id=reports_tabular_in_dashboards.htm&type=5