Final answer:
Filling out a Marketo form results in the creation of a new lead or updating of an existing lead in the system, avoiding duplicates and maintaining current account information.
Step-by-step explanation:
When a lead fills out a Marketo form, the action that occurs is A. A new lead is created or an existing lead is updated. Marketo forms are designed to capture information about individuals who are interested in a product or service. They are an essential component of lead management within Marketo's marketing automation platform.
Depending on whether the lead's information matches an existing record, Marketo will either create a new lead or update the existing one, thereby avoiding the creation of duplicate records and ensuring that accounts are kept up-to-date with the latest lead information.