Explanation:
The correct answer is option D, Word fields. Word fields are actually an essential component of performing a mail merge operation. The other three components mentioned, namely the main document, data source, and merge fields, are all necessary for a successful mail merge.

1. Main document: The main document is the template or the document that you want to personalize. It contains the static content that remains the same for each recipient. It may include text, images, and other formatting elements.

2. Data source: The data source is a file or a database that contains the recipient information. It can be an Excel spreadsheet, a Word table, an Access database, or any other structured data source. The data source provides the variable information that will be merged into the main document.

3. Merge fields: Merge fields are placeholders in the main document that indicate where the variable information from the data source should be inserted. They are typically enclosed in double curly brackets, such as {{FirstName}}, {{LastName}}, or {{Address}}. During the mail merge process, each merge field is replaced with the corresponding data from the data source for each recipient.

4. Word fields: Word fields are a different concept in Microsoft Word and are not directly related to mail merge operations. Word fields are used to insert dynamic content, such as dates, page numbers, or calculations, into a document. They are inserted using field codes, which are not typically used in mail merge operations.

In conclusion, Word fields are not an essential component of performing a mail merge operation. The main document, data source, and merge fields are the key elements required for a successful mail merge.