Final answer:
The invalid data source for a mail merge is a worksheetPDF because it is not a structured database or spreadsheet format.
Step-by-step explanation:
The correct answer is c. worksheetpdf. In a mail merge, a data source is used to populate the fields in a template document. The data source can be a database, an Excel spreadsheet, or a Word document. However, a PDF document cannot be used as a data source for a mail merge because it is not a structured database or spreadsheet format.