Answer: d) The "space race" was a Cold War competition by scientist.
Explanation: D is the best option because it puts "space race" as the subject of the sentence.
The subject is the main focus of the sentence.
Option A does this as well, but the sentence is very wordy and does not do enough to draw attention to "the space race".
Option B keeps the scientists as the subject of the sentence.
Option C makes the "space race" part of a prepositional phrase which can be eliminated.
Hence, Option D is the correct answer.
