Introduction:
In the mid-18th century, Europe underwent significant political and social changes. Diplomacy played a crucial role in maintaining peace and stability among different European nations. The language spoken for diplomatic purposes was crucial in facilitating communication between diplomats from different countries.

Answer:
The language spoken for the purpose of diplomacy in mid-18th century Europe was French.

Explanation:
The reasons for French being the language of diplomacy are as follows:

1. French was the language of the European elite: French was the language of the European aristocracy, and it was widely spoken among the elites of different European countries. This made French the most natural choice for diplomatic communication.

2. France was a dominant power in Europe: In the mid-18th century, France was a dominant power in Europe. French was the language of the French court and was used in diplomatic exchanges across Europe.

3. French was the language of culture and learning: French was considered the language of culture and learning in Europe. It was the language of literature, philosophy, and science. Diplomats needed to be well-educated and knowledgeable, and speaking French was a symbol of education and intelligence.

Conclusion:
In conclusion, French was the language spoken for the purpose of diplomacy in mid-18th century Europe. It was the language of the European elites, the dominant power, and culture and learning. These factors made French the most natural choice for diplomatic communication.

B) French