What does a good underline on a boar indicate?

A good underline on a boar is crucial because it indicates the potential for good mammary development in his offspring. This feature is particularly important in breeding programs where productive females are desired. A well-formed underline suggests that the boar carries genetic traits that can be passed on to females, which can lead to improved milk production and overall reproductive efficiency.

The other options do not accurately reflect the significance of a good underline. Faster growth potential is typically associated with overall body structure and size rather than underline quality. Strong bone structure can be assessed through other physical characteristics rather than the underline. Superior coat quality is important for overall health and appearance but is not related to the quality of the underline or its implications for progeny. Thus, option C stands out as the correct choice in the context of assessing a boar's breeding value.