• Sire Index: A mathematical estimator used in dairy cattle breeding to evaluate a bull's genetic transmitting ability for milk production based on the phenotypic performance of his daughters and their dams:
• Equal Parent Index (Intermediate Index):

where $\bar{D}$ is the average milk production of daughters and $\bar{M}$ is the average production of their dams.

• Represents the expected milk production that the sire would produce if he were a female expressing his own breeding value under standard herd conditions.
• Modern Counterparts: Largely modernized today into Predicted Transmitting Ability (PTA) and Estimated Breeding Value (EBV = $2 \times \text{PTA}$).

Correct option: (3) Sire Index