b)12
The tree maybe of depth 2 or 1. If depth is 2, we have 6 possible trees. This is because one of the 3 nodes A, B, C may be the root and the next level may be one of the remaining 2 nodes. If the depth is 1, the root maybe one of the 3 nodes A, B, C. Corresponding to a root, say A, 2 trees are possible as this.
A and A
B C C B
This gives us 6 more possibilities.
Total = $12$