Private Sub Worksheet_Change(ByVal Target As Range)
'Assume drop down in cell(1,1)
If Target.Row = 1 Then
'You need to change the option here
'You may also use select case statements instead of if statements
If Cells(1, 1).Value = "a" Then
Sheet2.Visible = xlSheetVeryHidden
Sheet3.Visible = xlSheetVisible
ElseIf Cells(1, 1).Value = "b" Then
Sheet3.Visible = xlSheetVeryHidden
Sheet2.Visible = xlSheetVisible
End If
End If
End Sub