To answer this question, let's review each option:
Option A) Validate - This option is incorrect because there is no attribute called "Validate" for a validator control.
Option B) ValidateControl - This option is incorrect because there is no attribute called "ValidateControl" for a validator control.
Option C) ControlToBind - This option is incorrect because there is no attribute called "ControlToBind" for a validator control.
Option D) ControlToValidate - This option is correct because the "ControlToValidate" attribute must be set on a validator control for the validation to work. This attribute specifies the ID of the control that should be validated.
Therefore, the correct answer is option D) ControlToValidate.