Final answer:
The vertebra is the bone classified as 'irregular' in shape due to its complex structure, fulfilling various roles that include support for the spinal cord.
Step-by-step explanation:
Irregular bones are a category of bones with complex shapes that don't fit into other classifications such as long, short, flat, or sesamoid bones. A key characteristic of an irregular bone is that it has a more complex structure with features that include short, flat, notched, or ridged surfaces. These special characteristics allow them to fulfill a variety of roles within the body, such as providing extensive protection or accommodating certain movements. Common examples of irregular bones include the vertebrae, several skull bones including those containing sinuses, and the bones of the pelvis.
Given the examples and definition, the answer to the question, "Which of the following bones is classified as 'irregular' in shape?" would be A) Vertebra. The vertebrae support the spinal cord and protect it from compressive forces, making it an irregular bone due to its complex shape and structure. The other options listed, namely, metatarsal, patella, and frontal bone do not fall under the category of irregular bones since metatarsals are long bones, the patella is a sesamoid bone, and the frontal bone is considered a flat bone.