Rebalancing a portfolio is the process of:

Rebalancing a portfolio specifically involves adjusting the proportions of different asset classes back to their predetermined strategic asset allocation. This predefined asset allocation reflects the investor's risk tolerance, investment objectives, and market conditions at the time of initial construction. Over time, as different assets within the portfolio experience varying degrees of performance, the actual allocation can deviate significantly from the desired levels.

By rebalancing, an investor ensures that the portfolio remains aligned with the intended risk-return profile, thus managing exposure to different asset classes and maintaining a disciplined investment approach. This process can help mitigate risks associated with overexposure to certain assets that have performed well and have thus increased in weight within the portfolio, while also providing opportunities to buy assets that may be undervalued after underperforming.

The other options focus on tactical changes or adjustments based on market momentum, which do not represent the systematic process of rebalancing back to a specific strategic asset allocation. Increasing the number of assets does not inherently relate to maintaining the existing strategy but rather could be part of diversification or different investment strategies altogether.