Final answer:
The default line spacing in Microsoft Word 2016 is Multiple 1.08 Spacing. For formatting business letters, block formatting with single spacing and double spacing between paragraphs is standard.
Step-by-step explanation:
The default line spacing in Microsoft Word 2016 is Multiple 1.08 Spacing. However, when formatting a business letter, it is standard to use block formatting, where the text is left-justified and single spaced, with double spacing between paragraphs. It is also recommended to use a legible font like Times New Roman or Arial in size 12, but these guidelines do not necessarily correlate with the default settings of Word 2016.