Place one of A, B, C or D into each of the 25 empty squares so that the number of letters in each row and column match the number indicated on that row and column for that letter.

Identical letters cannot be next to each other vertically or horizontally, but may be adjacent diagonally.

                 +---+---+---+---+---+
              A  | 2 | 2 | 0 | 1 | 1 |
                 +---+---+---+---+---+
              B  | 1 | 1 | 2 | 2 | 0 |
                 +---+---+---+---+---+
              C  | 2 | 1 | 1 | 0 | 1 |
                 +---+---+---+---+---+
  A   B   C   D  | 0 | 1 | 2 | 2 | 3 |
+---+---+---+---++===+===+===+===+===+
| 0 | 2 | 1 | 2 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+
| 1 | 1 | 2 | 1 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+
| 2 | 0 | 2 | 1 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+
| 2 | 1 | 0 | 2 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+
| 1 | 2 | 0 | 2 ||   |   |   |   |   |
+---+---+---+---++---+---+---+---+---+

---

From Mensa Puzzle Calendar 2019 by Fraser Simpson, Workman Publishing, New York. Puzzle for June 26.