Final answer:
Achondroplasia is a form of dwarfism; thus, the correct term to complete the sentence is 'dwarf,' resulting in the sentence: Betty was seen on campus with her friend Cathy, who was an achondroplastic dwarf.
Step-by-step explanation:
The question given is pertaining to a biological terminological identification. The term achondroplastic refers to achondroplasia, which is a form of dwarfism, a genetic disorder characterized by an abnormality in the conversion of cartilage to bone, particularly in the long bones of the arms and legs. This short-limbed disproportion is a common cause of disproportionate stature. Therefore, the correct answer to fill in the blank in the sentence would be option B. dwarf, making the full sentence 'Betty was seen on campus with her friend Cathy, who was an achondroplastic dwarf.'