The correct answer is: A. The father warned his son, “Beware of him!”
The other options are incorrect because they do not convey the same level of urgency or danger as the first option. Option B, “Watch that chap!”, is a more general warning that could be applied to any situation. Option C, “Be careful about him”, is also a more general warning, and it does not specifically mention that the son should be wary of the man. Option D, “Don’t fall into the trap”, is a warning about a specific action that the son should not take, but it does not specifically mention that the man is a threat.
The first option, “Beware of him!”, is the most direct and specific warning, and it conveys the sense that the man is a danger to the son. This is the warning that a father would most likely give to his son in this situation.