Final answer:
PI calves are persistently infected with Bovine viral diarrhea (BVD), a disease that causes health problems in cattle(option b).
Step-by-step explanation:
PI calves are persistently infected with the option b)Bovine viral diarrhea virus (BVD).
This means that calves are infected with the BVD virus for their entire life span and can spread the virus to other animals. BVD is a significant disease in the cattle industry and can lead to a range of health issues, including gastrointestinal problems, respiratory issues, and reproductive difficulties. It is critically important to manage and control BVD to maintain the health of cattle herds.