The document produced by knitr document has which of the following extension?

The correct answer is B. .rmd.

knitr is a R package that facilitates the production of dynamic documents. It can be used to create documents in a variety of formats, including HTML, PDF, and markdown. When knitr is used to create a document in markdown format, the output file will have the extension .rmd.

Option A is incorrect because .md is the extension for markdown files. Option C is incorrect because .html is the extension for HTML files. Option D is incorrect because it is not one of the possible extensions for knitr documents.