Careless of me!
You can add for further headers as required. Case else is adding the selected prefix to cells which do not contain a header identified in the Select Case code.
[vba]
Sub Prefix()
Dim i as long
Dim Pre as string
For i = 1 To Cells(Rows.Count, 1).End(xlUp).Row
Select Case Cells(i, 1)
Case "a"
pre = "Test 1 _ "
Case "b"
pre = "Test 2 _ "
Case "c"
pre = "Test 3 _ "
Case "d"
pre = "Test 4 _ "
Case "e"
pre = "Test 5_ "
Case Else
Cells(i, 1) = pre & Cells(i, 1)
End Select
Next
End Sub
[/vba]