Explanation:

- In the sentence, "These regions are very fertile," the word "fertile" means capable of producing abundant vegetation or crops.

- Option 1: sterile - means free from life or unable to produce crops. It can mean the opposite of "fertile."

- Option 2: arid - means dry or barren, often incapable of supporting vegetation. It contrasts with "fertile."

- Option 3: barren - means incapable of producing vegetation, essentially the opposite of "fertile." Correct Answer

- Option 4: productive - means able to produce or yield results or crops. It is synonymous with "fertile."