The correct answer is C. The realized return is calculated using the formula $\text{Realized return} = (1/T) \times \ln(\frac{S_T}{S_0})$, where $T$ is the time period in years, $S_0$ is the initial stock price, and $S_T$ is the stock price at the end of the period. Given $T = 0.5$ years (6 months), $S_0 = 280$, and $S_T = 320$, the realized return is $2 \times \ln(\frac{320}{280}) = 0.2671$ or 26.7%.

The annual volatility is calculated by scaling up the monthly volatility using the square root of time rule, which is $\text{Volatility per year} = \sigma \times \sqrt{12}$, where $\sigma$ is the monthly volatility. With a monthly volatility of 2.76%, the annual volatility is $0.0276 \times \sqrt{12} = 0.0956$ or 9.6%.

Option A is incorrect because it calculates the realized return as a simple percentage change rather than using the logarithmic return formula. Option B incorrectly assumes that annual volatility is simply 12 times the monthly volatility without applying the square root of time rule. Option D is incorrect for the same reason as B regarding the annual volatility calculation.