- `DataFrame.std()` calculates the standard deviation, so A matches with IV.
- `DataFrame.describe()` provides descriptive statistical values, so B matches with III.
- `DataFrame.var()` calculates variance, so C matches with I.
- `DataFrame.mode()` returns the value that appears most, so D matches with II.
Therefore, the correct answer is "A-IV, B-III, C-I, D-II."