Explanation

Option A is correct because:

• Quadratic programming is a mathematical optimization technique used in portfolio construction that allows for explicit risk control through parameter estimation
• It typically requires more inputs from market data (such as covariance matrices, expected returns, risk parameters) compared to simpler portfolio construction methods
• This method provides systematic risk control by incorporating risk constraints directly into the optimization process

Option B is incorrect because:

• Screening techniques typically focus on selecting stocks based on specific criteria (like P/E ratios, dividend yields, etc.)
• Concentrating stocks in selected sectors based on expected alpha actually increases sector concentration risk rather than providing superior risk control
• This approach can lead to unintended sector biases and higher portfolio risk

Option C is incomplete and incorrect because:

• The statement is cut off, but stratification involves dividing the market into segments (sectors, industries, etc.) and selecting stocks from each segment
• Risk control in stratification is typically achieved by maintaining proportional weights across segments, not by overweighting
• Overweighting certain segments would actually increase concentration risk

Quadratic programming remains the most sophisticated method among these for explicit risk control in portfolio construction, though it does require more complex inputs and computational resources.