Place an A, B, or C in each empty cell below so that no three consecutive cells (horizonally, vertically or diagonally) contain a set consisting either of all three different (A, B, and C) letters or all the same letter.
From Mensa Puzzle Calendar 2019 by Fraser Simpson, Workman Publishing, New York. Puzzle for December 20.
The filled out 4x4 grid is as follows:
+----------------------+
| A C A C |
| B B C C |
| B B A B |
| C A A B |
+----------------------+