In the Capital Asset Pricing Model (CAPM), which is closely associated with the Security Market Line (SML) approach, the expected return on a stock can be calculated using the following formula:
\[ \text{Expected Return} = \text{Risk-free rate} + \beta \times (\text{Market return} - \text{Risk-free rate}) \]
If a stock's beta is equal to zero, this means it has no correlation with market movements, and thus the additional risk premium associated with market fluctuations is not applicable to this stock. Therefore, the expected return simplifies to the risk-free rate.
So, the correct answer to the multiple choice question is:
The risk-free rate.