In the AfterUpdate event of that option control, you might use something
like (actual syntax may vary):
If Me.TheOptionControl = 1 Then 'use whatever # goes with "pending"
Me.YourDueDateControl = ... (whatever due date/expression you want
to assign)
Else
Me.YourDueDateControl = Null
End If
You will need to substitute the actual names of your controls.