Step 1: Understanding the Question:
The question asks us to identify the jurist who described the people as the "cause instrumentalis" (instrumental cause) and the state as the "cause a principalis" (principal cause) of law.
Step 2: Key Concepts and Approach:
This proposition comes from the Historical School of Jurisprudence, which focuses on how law develops from the popular consciousness (Volksgeist).
We need to identify which historical jurist modified Savigny's theory to give the state a more active role in the formulation of law.
Step 3: Detailed Explanation:
• Georg Friedrich Puchta: Puchta was a prominent disciple of Savigny and a key figure in the German Historical School.
• Modification of Volksgeist: While Savigny believed that law emerges solely and directly from the Volksgeist (spirit of the people), Puchta realized that the popular consciousness is too abstract to form a complete legal system.
• Role of the State: Puchta argued that while the people (Volksgeist) are the ultimate source or "cause instrumentalis" of law, the state must give formal shape and enforcement to these rules, making the state the "cause a principalis" of law.
• Systematization: Puchta’s synthesis helped bridge the gap between purely historical development and the practical reality of state-enforced legal codes.
Step 4: Final Answer:
This specific proposition was laid down by Georg Friedrich Puchta. Thus, Option C is the correct answer.