What should you do if the add to campaign button is disabled in one of your reports?

Answer options:

A.Modify your report to return fewer than 100,000 records
B.Modify your report to return fewer than 150,000 records
C.Modify your report to return fewer than 50,000 records
D.None of the above

Correct Answer: C 
If the add to campaign button has been disabled in one of the reports then the report needs to be modified so that it can return less than 50,000 records. 
Other options are incorrect because they don’t meet the value being less than 50,000 records. 
Reference: 

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