Step 1: Understanding the Doctrine of Monads.
The Doctrine of Monads, proposed by Leibniz, suggests that the universe is made up of simple substances called monads. These monads are indivisible, and each one reflects the entire universe in its own way. The idea that "Body is a plurality of simple forces" aligns with the notion of monads, where each force or substance operates as a simple unit but interacts with others in complex ways.
Step 2: Analyzing the Options.
- 1. Mind-Body dualism: This doctrine, associated with René Descartes, argues that mind and body are two separate substances. It does not discuss forces in the way described in the question.
- 2. Categorical Imperative: This is a moral principle proposed by Immanuel Kant, focusing on duty and ethics, not the nature of forces.
- 3. Doctrine of Modes: This doctrine, associated with Spinoza, refers to the modes or modifications of substance, not directly about forces or bodies.
- 4. Doctrine of Monads: This is correct. Leibniz’s Doctrine of Monads directly addresses the idea of simple substances or forces that combine to form complex bodies.
Step 3: Conclusion.
The correct answer is 4. Doctrine of Monads.
Final Answer:
\[
\boxed{\text{The correct answer is 4. Doctrine of Monads.}}
\]