Final answer:
Option (a), John Locke believed that the government should protect natural rights and exist by consent of the governed, while Montesquieu advocated for a balanced constitutional monarchy.
Step-by-step explanation:
The Enlightenment thinker who argued that all people are born free in a state of nature, and the government should exist only by their consent, is John Locke. Locke's view of government centered around the protection of natural rights, particularly life, liberty, and property. He believed that the purpose of government is to serve the people and that it must operate with the consent of the governed.
This concept is known as popular sovereignty. Conversely, Baron de Montesquieu advocated for a form of government with a separation of powers, a constitutional monarchy, where power is shared and balance is essential to prevent tyranny.