Answer:
B. In the early years of the space program, astronauts studied three complex subjects: rocket engines, flight mechanics, and computer theory.
Step-by-step explanation:
The correct sentence is sentence B, "In the early years of the space program, astronauts studied three complex subjects: rocket engines, flight mechanics, and computer theory.
A colon is often used before a list. However, it is important to remember that the part of the sentence before a colon needs to be a complete sentence. We could just say, "In the early years of the space program, astronauts studied three complex subjects," is a complete sentence that could stand alone.