Many snakes are innocuous.

The correct answer is A. poisonous.

Innocuous means not harmful or dangerous. Poisonous means capable of causing death or illness by the ingestion, inhalation, or contact with a poison. Harmful means causing or likely to cause harm. Deadly means causing or capable of causing death. Ferocious means fierce or savage.

Of the four options, only poisonous is a synonym for innocuous. Harmful, deadly, and ferocious are all words that can be used to describe snakes, but they do not mean the same thing as innocuous.