The correct answer is D) Foam. Foam is the recommended extinguishing agent for a grade B product of 1,3-Pentadiene. Grade B fires involve flammable liquids and gases, and foam is an effective agent for extinguishing these types of fires. Foam works by creating a blanket that separates the fuel from the oxygen, smothering the fire. The other options, such as high or low velocity fog or water, would not be as effective in extinguishing a grade B fire involving a flammable liquid like 1,3-Pentadiene.