Final answer:
The combining form 'osteo-' refers to a part of the human skeleton that can be touched.
Step-by-step explanation:
The correct answer is a) Osteo-. The combining form 'osteo-' refers to bones. 'Myelo-' refers to bone marrow, 'chondro-' refers to cartilage, and 'cutaneo-' refers to skin. So, out of the given options, 'osteo-' is the combining form that refers to a part of the human skeleton that can be touched.