We are asked to find the correct verb form of the word 'poison'.
The word 'poison' is used as a noun in its basic form, but the verb form is simply 'poison' (without any alteration). This verb form means to administer poison to someone or something.
- Option 1, 'poisonous', is an adjective.
- Option 2, 'imprison', is a verb, but it is unrelated to the meaning of 'poison'.
- Option 3, 'poisons', is the third person singular form of the verb, but it's not the base form.
- Option 4, 'poison', is the correct base verb form.
Thus, the correct answer is \( \boxed{poison} \).