Can i select the columns with Table.SelectColumns based on a condition - M Code?

Yes. You need to convert the A to 1, B to 2, etc. You can do that with the following assuming your letter is in the Code column:

=Character.ToNumber([Code])-64

The following M code imports a 9 column Excel table, adds the Char Code (above) then gives you the column name of the column referred to by the code.

let
    Source = Excel.CurrentWorkbook(){[Name="Table4"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Code", type text}, {"Column A", type text}, {"Column A2", type text}, {"Column A3", type text}, {"Column A4", type text}, {"Column A5", type text}, {"Column A6", type text}, {"Column A7", type text}, {"Column A8", type text}}),
    #"Added Char Code" = Table.AddColumn(#"Changed Type", "Char Code", each Character.ToNumber([Code])-64, Int64.Type),
    #"Added Custom1" = Table.AddColumn(#"Added Char Code", "Relative Column Name", each Table.ColumnNames(#"Added Char Code"){[Char Code]})
in
    #"Added Custom1"