Answer:
C. possible
Step-by-step explanation:
"The writer’s perspective is that long-term space travel is not an immediately viable or even desirable proposition. It is necessary for the writer to concede the point expressed in sentence 12 that NASA is making a concentrated effort to solve problems associated with long-term space missions; stating that solutions are “possible” helps to convey the writer’s skepticism whether all or even many of the problems will be solved." - online assignment