The correct answer is option 'C': philosophers such as John Locke and Rousseau.

Explanation:
The advocacy for a society based on freedom, equal laws, and opportunities was primarily promoted by philosophers such as John Locke and Rousseau. These philosophers played a significant role in shaping the ideals of modern democratic societies. Here is a detailed explanation of their contributions:

1. John Locke:
- John Locke was an influential Enlightenment thinker who advocated for individual rights and limited government.
- He believed that all individuals are born with natural rights, including life, liberty, and property.
- According to Locke, the primary role of government is to protect these rights, and if a government fails to do so, people have the right to rebel against it.
- His ideas are reflected in the concept of a social contract, where individuals willingly give up some of their freedoms in exchange for protection and the rule of law.

2. Jean-Jacques Rousseau:
- Rousseau was another prominent Enlightenment philosopher who contributed to the idea of a society based on freedom and equal opportunities.
- He believed in the concept of the general will, which suggests that decisions made by a society should reflect the collective interests and welfare of its members.
- Rousseau argued for the establishment of a social contract where individuals would willingly surrender some of their rights to the community for the common good.
- He also emphasized the importance of education in creating a just and equal society, advocating for universal education for all citizens.

The ideas put forward by Locke and Rousseau greatly influenced the political and social landscape of the time. Their writings and philosophies inspired the French Revolution and the American Revolution, which aimed to establish societies based on the principles of liberty, equality, and justice.

Other options mentioned in the question are not correct:
- Option 'A' (middle class and people of the Third Estate) is incorrect because while these groups may have supported the ideals of freedom and equal opportunities, they did not advocate for them as explicitly as the philosophers did.
- Option 'B' (clergy and nobility) is incorrect because these groups were often resistant to change and did not actively promote ideas of freedom and equal opportunities.
- Option 'D' (Englishmen Georges Danton and Arthur Young) is incorrect because their contributions were not central to the advocacy of a society based on freedom, equal laws, and opportunities.