How can a user keep field values hidden when using them in a formula?

Marking the field as hidden is the most effective way to ensure that field values remain concealed when utilized in a formula. This feature allows the user to execute calculations or functions using the data without exposing its actual contents to the end user. By designating the field as hidden, users can maintain data privacy and security, thereby preventing visibility of sensitive information in reports or interfaces while still leveraging the data for necessary analytical purposes.

The other options, while they may modify the field in some way, do not specifically address the need to keep the field values hidden from users. Changing the data type could affect how the data is processed but does not provide any concealment of the values themselves. Renaming the field changes its title but does not guard the original data. Making the field a key column impacts the way the data is structured and indexed but does not hide its contents in computations. Therefore, marking the field as hidden is the most targeted approach to achieving the desired secrecy around field values in a formula context.