Step 1: Understanding the Question:
The question asks us to identify the person who *originally* propounded the theory of "social solidarity."
Step 2: Key Concepts and Approach:
We must distinguish between the sociologist who originally created the concept of social solidarity and the legal jurist who later applied it to legal theory.
Step 3: Detailed Explanation:
• Émile Durkheim: Durkheim, a founding French sociologist, originally propounded the concept of "social solidarity" in his famous work "The Division of Labour in Society" (1893). He described how organic and mechanical solidarity hold modern societies together.
• Leon Duguit’s Adaptation: Leon Duguit, a French constitutional jurist, later adapted Durkheim’s sociological concept to jurisprudence. Duguit argued that the primary purpose of law is to promote and maintain social solidarity.
• Distinguishing the Two: While Duguit is highly famous in jurisprudence for this theory, Durkheim is the *original* propounder of the concept.
Step 4: Final Answer:
Because the question asks who *originally* propounded the theory, the correct answer is Émile Durkheim. Hence, Option C is correct.