According to Basel 2.5, Stressed VaR is calculated by identifying a one-year period (equivalent to 250 trading days) from the most recent seven years that was most stressful for a bank's current portfolios. This period is identified based on the bank's current risk profile and the historical market conditions during that year. The Stressed VaR is then calculated using the risk factors and market data from this identified stressful period. This approach ensures that the bank's capital requirement is sufficient to cover potential losses under extreme market conditions similar to those experienced during the identified stressful year.

Choice A is incorrect. Stressed VaR is not calculated by multiplying 1-day VaR from the recent daily variation in values by $\sqrt{10}$. This method does not take into account the stressed market conditions which are a key aspect of Stressed VaR calculation as per Basel 2.5.

Choice C is incorrect. While it's true that Stressed VaR involves selecting a period of stress, according to Basel 2.5, this period should be one year from the most recent seven years that exhibited stress in its current portfolio, not ten years as stated in this option.

Choice D is incorrect. As explained above, option B correctly describes the method of calculating Stressed VaR as per Basel 2.5, hence 'None of the above' cannot be correct.