Which of the following bones is NOT a long bone: a) Humerus, b) Sternum, c) Tibia, d) Ulna?

A long bone is characterized by a shaft (diaphysis) and two expanded ends (epiphyses), and it is typically longer than it is wide.

• a) Humerus: This is the long bone of the upper arm.
• b) Sternum: This is the breastbone, located in the center of the chest. It is classified as a flat bone, not a long bone.
• c) Tibia: This is the larger of the two bones in the lower leg (shin bone). It is a long bone.
• d) Ulnar: This refers to the Ulna, one of the two long bones in the forearm.

Therefore, the sternum is not a long bone.

The final answer is $\boxed{\text{b) Sternum}}$.