A 2-3 tree only allows up to 3 children, which means a node can only have at most 2 values. (b) has an extra value in the rightmost node.
A 2-3 tree must also be perfectly balanced. (c) is imbalanced.
A B-Tree node always has one more child than the number of values. So in a 2-3 tree, a node with 1 value has 2 children, and a node with 2 values has 3 children. (d) violates this with the node 21, which has one value but 3 children.
Procedural
Draw the 2-3 tree that results from inserting 1, 2, 3, 4, 5, 6, 7 in order.
