Final answer:
Option (B), Sexual selection is the force that operates on male song sparrows, leading to the evolution of traits that enhance mating and reproductive success.
Step-by-step explanation:
The selection that operated on male song sparrows is sexual selection. This is a specific type of natural selection that affects an individual's ability to mate and reproduce. It can result in the evolution of traits that may seem maladaptive for survival but give individuals greater reproductive success. In the case of song sparrows, traits such as song quality, which are used in either male-male competition or are preferred by females during mate choice, are favored.
Sexual dimorphism, where males and females of a population look or act differently, is often a result of sexual selection. This can be seen in many bird species, including the song sparrows, where males may have more vibrant feather colors or more complex songs than females, specifically because these traits are selected by females during the mating process.