Universal Container Sales Ops director requested from the salesforce admin to have a field on the account page layout to be able to track the number of licenses being sold for the company mobile application app per account. How can the salesforce admin achieve this request?

Answer options:

A.Create Validation Rule
B.Create Formula field
C.Create Roll-Up summary field
D.Create Text field

Correct Answer: C 
Using the roll-up summary field, the salesforce admin will be able to get values on a master records from a child record. So, it can calculate values from related records. 
Option A is incorrect because validation rule doesn’t have the option to do any field update. 
Option B is incorrect because formula fields calculate values using fields within single record. 
Option D is incorrect because the text field needs to be filled in with a value and it doesn’t have any automation. 

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