Answer: Partitions of n-element set into k non-empty subsets.
- A Arrangements of n objects that have exactly k fixed points
- B Partitions of n-element set into k non-empty subsets
- C The number of k-element combinations chosen from n objects
- D n choose k, the ordinary binomial coefficient
Correct answer: B. Partitions of n-element set into k non-empty subsets
Explanation: Stirling numbers of the 2nd kind S(n,k): partition a set of n distinct elements into exactly k non-empty, unordered subsets. E.g. S(3,2)=3: {1},{2,3} and permutations.
Counting tree for selecting 2 items from {A, B, C} without repetition: 3 x 2 = 6 ordered arrangements.
Concept context
Counting, arrangements, and selections