A) varied
Varied/variety means there's many, which is said in the next sentence.
MLA 8 was designed to simplify the process, helping writers accurately and intuitively cite sources more easily, requiring that every source type follow the same format. This means that books, websites, periodicals, videos, photographs, and all other types of sources now use this same standard format.
The answer is: [D]: "something that can be proven accurate" .
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