To rewrite 812 using a root, you could express it as follows:
\(\sqrt{812}\)
However, if the intention is to simplify or break it down further, we could also express it in a format that shows its prime factors:
\(\sqrt{4 \times 203} = 2\sqrt{203}\)
But choosing from the responses you provided, it seems that none of them correctly express or simplify 812 as a root. If the goal is to physically show "812" as part of a square root expression, then \(\sqrt{812}\) would be the most appropriate notation.