A medicine to counteract poison is called an 'Antidote'.Thus, the correct option is A.

Option B: This refers to something related to water, particularly plants or animals that live in water. It has no connection to counteracting poison.

Option C: This refers to someone who believes that the existence of God or a higher power is unknown or unknowable. It is unrelated to medicine or poison.

Option D: This is the study of human societies, cultures, and their development. It is not related to medicine or counteracting poison.