Hi Dave,

not simple, IMHO.

You may set the font of the range of a checkbox to hidden.
But for that you have to remove the protection.
Removing the protection resets the values of fields
to their default values.
Therefore it seems, you have to remember the values
(results) of the fields and restore them after
changing the font.hidden property.

Sub MyExit67003()
Dim sTmp As String
Dim bChk As Boolean
With ActiveDocument
  sTmp = .FormFields(3).Result
  bChk = .FormFields(2).CheckBox.Value
  .Unprotect
  If sTmp = "a" Then
     .FormFields(2).Range.Font.Hidden = True
  Else
     .FormFields(2).Range.Font.Hidden = False
  End If
  .Protect wdAllowOnlyFormFields
  .FormFields(3).Result = sTmp
  .FormFields(2).CheckBox.Value = bChk
  MsgBox .FormFields(2).Result ' for testing
End With
End Sub

I don't think I'm an expert on formfields and forms.
If you have a close look, you'll notice,
that I didn't bother about formfields(1).

But if it helps, its alright.