Which of the following is an antidote for a blood agent?

Amyl nitrate is recognized as an antidote for blood agents, particularly cyanide. Blood agents are toxic substances that affect the blood's ability to transport oxygen, leading to cellular suffocation. Amyl nitrate works by converting hemoglobin to methemoglobin which binds to cyanide, thereby helping to alleviate the poisoning effect and promoting the excretion of cyanide from the body. This mechanism allows for the restoration of the blood's oxygen-carrying capacity, making it a critical treatment in scenarios involving exposure to blood agents.

The other options, while they may have medical uses, do not serve as antidotes specifically for blood agents. Sodium bicarbonate is mainly used to treat acidosis and has no direct effect on blood agents. Atropine is effective against nerve agents and certain types of chemical weapon exposure, rather than blood agents. Diazepam is a sedative that can be used to manage seizures and anxiety but is not an antidote for blood agents. Understanding the specific uses of these treatments is essential for effective medical response in cases of chemical exposure.