Assuming the validation rules are the same for C4:C34 / F4:F34 as for B2/D2 then see below:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim boolOK As Boolean
If Target.Count > 1 Or Intersect(Target, Range("B2,C4:C34")) Is Nothing Then Exit Sub
Select Case Val(Target.Offset(, 2 - (Target.Column = 3)).Value)
Case Is <= 50
boolOK = UCase(Target.Value) = "NO"
Case Else
boolOK = UCase(Target.Value) = "YES"
End Select
Application.EnableEvents = False
If Not boolOK Then Target.Offset(, 2 - (Target.Column = 3)).Value = ""
Application.EnableEvents = True
End Sub
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