Option D.French priests believed in the concept that people were members of one of the three orders .The three orders were three social categories:Christian priests, land owning nobles and peasants.They were categorised depending on their work.The changing relationships between these three groups was an important factor in shaping European history for several centuries.

The correct answer is option 'D', which states that the basis of classification among the three orders was the nature of work. Let's delve into this further and understand the reasoning behind it.

The three orders, also known as estates or classes, were a social and political structure that existed in France before the French Revolution. These orders were the First Estate, comprised of the clergy and religious leaders; the Second Estate, consisting of the nobility and aristocracy; and the Third Estate, which included commoners, peasants, and the bourgeoisie.

The classification of individuals into these orders was primarily based on the nature of their work or occupation. This categorization was deeply ingrained in the feudal system, where one's social status and privileges were determined by their role in society. Here's a breakdown of the basis of classification:

1. First Estate (Clergy):
- The clergy, mainly priests and religious leaders, belonged to the First Estate.
- Their primary occupation was to serve the Church and attend to religious matters.
- They held significant power and influence in society, often enjoying exemptions from certain taxes and other privileges.

2. Second Estate (Nobility):
- The nobility, including feudal lords, aristocrats, and noble families, constituted the Second Estate.
- Their primary occupation was to govern and protect the land under their control.
- They held hereditary titles and owned large estates, enjoying privileges and exemptions from various obligations.

3. Third Estate (Commoners):
- The Third Estate encompassed the majority of the population, including commoners, peasants, merchants, artisans, and the emerging bourgeoisie.
- Their occupations varied greatly, ranging from farming, trade, craftsmanship, and labor.
- They faced heavy taxation, lacked significant political power, and had limited social mobility compared to the upper classes.

In summary, the nature of work or occupation formed the basis of classification among the three orders in pre-revolutionary France. This classification system reinforced social hierarchies, with the clergy and nobility enjoying privileges and power while the commoners faced numerous challenges and limited opportunities for advancement. The discontent arising from these inequalities played a significant role in fueling the French Revolution, as the Third Estate sought to overthrow the existing social order and demand greater rights and representation.