Final answer:
Language development in adulthood varies widely, principally due to a person's level of education, as well as their social and occupational roles, influencing language skills and usage.
Step-by-step explanation:
Language development during adult years varies greatly among individuals depending on several factors. The correct answer to the student's question is c. level of education and social and occupational roles. These factors are pivotal as they dictate the exposure to various vocabulary, social interactions, and professional terminologies that influence language proficiency and use. Additionally, everyday communication and identity expression through language demonstrate the fluidity of language use, depending on the setting, audience, and societal norms. Research supports the idea that our language development is influenced not only biologically but also by social and cultural factors, and continues to change throughout adulthood.