Explanation:

- Let's first drop the first digit of each number:

- 243 becomes 43

- 874 becomes 74

- 923 becomes 23

- 456 becomes 56

- 721 becomes 21

- Now interchange the second and third digits of the original number to get:

- 243 becomes 34

- 874 becomes 47

- 923 becomes 32

- 456 becomes 65

- 721 becomes 12

- Compare the new numbers:

- 34

- 47

- 32

- 65

- 12

- The highest number in this set is 65.

- Correct Answer: Option 4 - 456