Answer:
Countries.
Step-by-step explanation:
A simple subject is a single noun or pronoun that occurs usually before the verb contrary to the complete subject which may contain modifiers like an adjective, prepositional phrases, etc. A simple subject is free from modifiers and generally refers to one specific idea, person, or thing. In the given sentence, the noun that illustrates and justifies this claim is "Countries" as it answers 'who' performs the subject.