Final answer:
Forage sorghum is the type of forage where prussic acid is of greatest concern to cattle, due to its potential to produce cyanogenic glycosides. Therefore, the most appropriate correct option is D.
Step-by-step explanation:
Prussic acid, also known as hydrogen cyanide, is a concern for cattle when they consume certain types of forages. Among the options provided, forage sorghum is the type of forage that is of the greatest concern for the presence of prussic acid. Forage sorghum may produce cyanogenic glycosides which can be converted into prussic acid, especially under stress conditions such as drought or frost. These compounds can be toxic to cattle if ingested in large quantities.