To dynamically display a field based on the state of a checkbox in a Pega application, the correct approach is:

D. Use a when condition in the UI form to determine whether to show the mobile phone number field. This configuration involves setting a visibility condition directly on the mobile phone number field in the form, using a "when" rule. The rule evaluates the state of the checkbox, and if selected, the field for entering a mobile phone number becomes visible. This solution leverages Pega's dynamic UI capabilities to show or hide fields based on user interactions without needing server-side interventions like data transforms or flow changes.