LD50 is a measure of toxicity in animals. A chemical with a smaller LD50 is more toxic and in this question, Shiga toxin has a lower LD50, thus it is more lethal.
The answer is mitochondria because this is the super producer of cell systems. while an active animal, (squirrel, rodents, horses, deer etc.) are all prey items for preditors, they possess more energy to survive & escape danger...