Hi, grkchakri,
if you want to to hard-code the brand names you may work like this:
Private Sub RAN_NUMB_Click()
Dim RN1 As Long
Dim RN2 As String
Dim RN3 As Range
Dim RN4 As Range
'MsgBox ("Create Random Numbers for " & vbNewLine _
& "1.Months" & vbNewLine _
& "2.Product Code" & vbNewLine _
& "3.No of Items" & vbNewLine _
& "4.Sales")
RN2 = InputBox("ENTER YOUR CHOICE" & vbNewLine _
& "1.Months" & vbNewLine _
& "2.Product Code" & vbNewLine _
& "3.No of Items" & vbNewLine _
& "4.Sales")
Select Case RN2
Case 1 'Months
For Each RN3 In Range("b2:b15")
RN3.Value = MonthName(Int((10 - 1 + 1) * Rnd + 1), True)
Next
Case 2 'product Codes
For Each RN3 In Range("c2:C15")
RN3.Value = "PC00" & (Int((20 - 1 + 1) * Rnd + 1))
Next RN3
For Each RN4 In Range("D2:D15")
Select Case RN4.Offset(0, -1).Value
Case "PC002"
RN4.Value = "Toyota"
Case "PC003"
RN4.Value = "Honda"
Case Else
RN4.Value = RN4.Offset(0, 1).Value
End Select
Next RN4
End Select
End Sub
I´d suggest you work with a table and use WorksheetFnction.VLookUp to deliver the brand name. 
Ciao,
Holger
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