The correct answer is D) For portable extinguishers, the agent is only available in a 2 pound unit. Halon 1301 is a highly effective fire extinguishing agent that works by interrupting the chemical chain reaction of the fire. It is commonly available in portable extinguishers, and the standard size for these units is 2 pounds. The other answer choices are incorrect - Halon extinguishes fires through a chemical mechanism, not by smothering (A), it can be used on electrical fires without leaving a residue (B), and it is not highly toxic at normal room temperature (C).