Step 1: Understanding Socrates' theory of virtue.
Socrates' philosophy on virtue is based on the belief that virtue is knowledge and that true knowledge leads to virtuous action:
- A. Knowledge is Virtue: Socrates famously asserted that virtue is a form of knowledge and that people do wrong only because they are ignorant of what is truly good.
- B. Virtue is One: Socrates believed that all virtues are essentially one, a unified form of excellence in character.
- C. Knowledge is Power: This concept is more associated with Francis Bacon, not Socrates.
- D. Virtue is a Unity: This is consistent with Socrates' belief that all virtues are interconnected and part of a unified whole.
Step 2: Conclusion.
The correct answer is 2. A, B and D only.
Final Answer:
\[
\boxed{\text{The correct answer is 2. A, B and D only.}}
\]