The answer: Brain
In Angie's situation, she has her spinal cord intact but still has difficulty walking. Medically, balance, gait and walking require complete functions of the brain and spinal cord. Should her spinal cord and its nerves be intact, the problem lies with the other important factor which is the brain. The brain sends the signals to do any activity.
D. gerund phrase as a direct object
Gerund is one of three kinds of verbals. Like all verbals, gerunds are nouns derived from verbs. Gerunds end in -ing. In the sentence, "leading" is the gerund. This gerund phrase is used as the direct object.
Tom feels like he is of the "superior" race. He explains a book that he read that says that if they don't watch out, the white race will be "submerged" in just a matter of years. Clearly he believes that as a white man he is of the dominant race, and the fact that there is a book published about it and he can freely say these things shows that many people in society are of the same mind.
Some people today still think this way, although it is socially not acceptable to say it or act like it. Fitzgerald probably included this information to show how opinionated Tom is and set the tone of society at the time.
Giving and spending time with family!