You create a custom field type and a CustomFieldControl.ascx user control.

You need to write the code-behind of the CustomFieldControl.acsx user control.

Which object should you override?

A.
BaseFieldControl

B.
SPFieldCalculated

C.
SPFieldText

D.
WebPart

Explanation:
MNEMONIC RULE: “CustomFieldControl = BaseFieldControl”

Renders a field on a form page (not a list view page) by using one or more child controls such as a label, link, or text box control.

BaseFieldControl Class
http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.webcontrols.basefieldcontrol.aspx