Step 1: Kant's Epistemology.
Immanuel Kant posited that knowledge stems from two primary faculties: sensibility (the capacity to receive sensory input) and understanding (the faculty for structuring and conceptualizing this input). He also highlighted reason's role in integrating knowledge. Notably, Kant did not regard verbal testimony as a valid epistemological source, as it relies on external authority rather than direct experience or logical derivation.
Step 2: Evaluating the Options.
- 1. Understanding: Valid. Kant considered understanding essential for knowledge acquisition, processing sensory data. - 2. Sensibility: Valid. Sensibility, the faculty of sensory perception, is another core source of knowledge for Kant. - 3. Reason: Valid. Reason is fundamental to Kant's philosophy, enabling knowledge synthesis and the application of categories to empirical data. - 4. Verbal Testimonies: Invalid. Verbal testimonies are not a source of knowledge within Kant's system, as they depend on external claims, not personal experience or rational inference.
Step 3: Conclusion. The correct option is 4. Verbal Testimonies.
Final Answer: \[ \boxed{\text{The correct answer is 4. Verbal Testimonies.}} \]