Final answer:
Pachycephalosaurus were likely bipedal due to their long hind legs and short forelimbs, which supported locomotion on two legs rather than four. Their not-tall stature is consistent with bipedalism; thus, all given reasons contribute to this belief.
Step-by-step explanation:
It is believed that Pachycephalosaurus were bipedal, meaning they walked on two legs, for several reasons. Firstly, their hind legs were long, which would have made them well-suited for walking or running on two legs. Additionally, their forelimbs are too short to have been used for walking, indicating that these limbs were not bearing the body's weight during locomotion. Also, while being not particularly tall dinosaurs, their body structure still favored bipedalism. Therefore, the correct answer is d. All of the above.