Typically the annualised standard deviation of log returns, using the square-root-of-time rule with sqrt(252) for daily data.
The central options trade is realised versus implied: a delta-hedged long option position profits when realised exceeds the implied volatility paid, and loses otherwise.
Estimation choices matter. Close-to-close estimators discard intraday information; Parkinson and Garman-Klass estimators use high-low ranges and are substantially more efficient for the same sample. Knowing that these exist is a good detail.