The correct answer is false.
It is false that Nahum prophesied against Israel over a period of 15 years.
What is true is that Nahum always had words of solace, compassion, and comfort to Judean people with his series of prophecies. Naum, the Jewish prophet, predicted the fall of Thebes, an important city of the Egyptian Empire. This happened in 663 BC. Nahum also predicted the fall of Nineveh, the capital of the Assyrians, in 612 BC. Historians agree that prophet Nahum wrote their prophecies thinking of giving hope messages to the Jewish people.