Final answer:
To trigger a Smart Campaign when a new lead enters the system by filling out a specific form, use the 'Lead is Created' trigger and add the Form Name constraint.
Step-by-step explanation:
To trigger a Smart Campaign when a new lead enters the system by filling out a specific form, you should use:
C) The Lead is Created trigger and add the Form Name constraint to pick the exact form.
This option allows you to specify the trigger for the Smart Campaign as the creation of a new lead, and then add the constraint of the specific form name you want to trigger the campaign.