Final answer:
Valid data types for a custom formula field include Date, Currency, Email, and Percent. Radio button is not a data type; it is a form element for user input.
Step-by-step explanation:
The question asks about a valid data type for a custom formula field. In the context of customizable databases or platforms like Salesforce, the following data types can be used for custom formula fields: Date, Currency, Email, and Percent. Each of these data types serves a different purpose in a formula field to manipulate and present data dynamically based on other values in the database. For example, a Date data type holds information about a specific calendar date, Currency handles monetary values, Email data type is used for email addresses, and Percent can represent values as a percentage. However, Radio button is not a data type; rather, it's a user interface element that allows users to select one option from a set.