Kennedy's argument is that the study of medicine in space will make everyone's lives healthier and happier.
Answer : B. some of the police force
Among the choices above, it is the phrase in letter b wherein the pronoun "some" refers to a single noun which is " the police force". As you can see, the police force refers to a group of policemen making it a collective noun taking the singular form of the verb. The other objects of the pronoun "some" in the other options are all in plural form such as economy cars, native people, and the 6y foods.
The correct answer would be the third option, prosperous. This means it's successful and doing well :)