Answer: 5.
- A 5
- B 14
- C 4
- D 6
Correct answer: A. 5
Explanation: C₃ = C(6,3)/(3+1) = 20/4 = 5. Verify: C(6,3) = 6!/(3!3!) = 720/36 = 20. Catalan numbers count valid bracket sequences, triangulations, etc.
Counting tree for selecting 2 items from {A, B, C} without repetition: 3 x 2 = 6 ordered arrangements.
Concept context
Counting, arrangements, and selections