ESG portfolio optimization involves incorporating ESG factors into the portfolio construction process. This process typically requires setting specific constraints or targets related to ESG variables to ensure the portfolio aligns with sustainability objectives.

Defining upper and lower bounds (C): This approach involves setting limits for specific ESG variables, such as carbon emissions or governance scores, either in absolute terms or relative to a benchmark. These bounds help to optimize the portfolio by ensuring it meets predefined ESG criteria while still aiming for financial performance.

Targeting sustainability-aligned themes (A): While targeting specific themes can be part of the strategy, it is not the core process of optimization, which focuses on balancing ESG constraints with financial objectives.

Applying a fixed decision on specific securities (B): This approach is more rigid and does not offer the flexibility required for portfolio optimization, which seeks to balance various factors and constraints.