Explanation

In a multifactor model, the inputs typically include:

• Factor betas (B): These measure the sensitivity of an asset's returns to each factor
• Deviation of factor values from their expected values (C): This represents the unexpected component of factor movements that drives returns
• The mean-variance efficient market portfolio (A): In some multifactor models (like the APT), this serves as a reference portfolio

Firm-specific returns (D) are NOT inputs to a multifactor model - they are the OUTPUT or residual component that represents the portion of returns not explained by the common factors. The multifactor model aims to explain asset returns using systematic factors, and the firm-specific returns are what remains after accounting for these factor exposures.

Therefore, firm-specific returns are least likely to be an input to the model.