Answer: A. Validation rule
Explanation:
Validation rules can be used to ensure that fields are populated because they can verify if the data being inputted meet the requirements of the administrators by returning a 'True' or 'False' response after testing. If the result is false, the user will be unable to move on until they adjust the field.
For example, Field length checks on websites are a Validation rule where you are only allowed to input a set number of characters and anything less or more will be denied, such as in the case of inputting phone numbers.
By including a Validation Rule that the LeadSource field of the Lead record be populated when a Lead is converted, the developer will be able to make sure that a user populates the Lead Source field prior to converting a Lead.
