Which of the following is the unit of issue for board foot?

The unit of issue for board foot is represented by the abbreviation "BF." A board foot is a volumetric measure of lumber that represents a one-foot length of a board that is one foot wide and one inch thick. When purchasing or selling lumber, the board foot measurement is a standard unit that allows for a consistent way to communicate quantity. The use of "BF" is widely recognized in the woodworking and building industries, making it the correct choice for this question.

The other choices do not correspond to the established abbreviation for board feet. "BL," "BO," and "PM" are not standard units associated with lumber measurement. Thus, "BF" is the only accurate representation among the options provided.