Final answer:
Cyanogenic toxin, resulting from eating certain improperly processed foodstuffs like cassava, can cause the blood to be dark brown due to cyanide poisoning.
Step-by-step explanation:
The type of toxin that comes from eating cereals, grasses, and root crops and causes the blood to be dark brown is A) Cyanogenic toxin. This toxin contains compounds that can lead to cyanide poisoning. Cyanide is a potent inhibitor of an essential step in aerobic metabolism known as oxidative phosphorylation, where it inhibits cytochrome c oxidase (COX). Consumption of cassava, which contains cyanogenic glycosides, can result in cyanide poisoning if the cassava is improperly processed. Inhalation of smoke from a house fire is another common route of exposure to cyanide.