Final answer:
The instantaneous speed halfway through the motion of the Subway train, which moves at a constant speed of 10 m/s, is 10 m/s since the speed is constant throughout the motion.
Step-by-step explanation:
The question pertains to the concept of instantaneous speed in physics. Given that a Subway train moves at a constant speed of 10 m/s over a period of 10 s, the instantaneous speed at any point during this motion is the same as the constant speed. Therefore, halfway through this motion, which is at the 5-second mark, the instantaneous speed is also 10 m/s. This is due to the fact that when an object moves at a constant speed, its instantaneous speed at any given moment during the motion does not change. Thus, the correct answer to the question is A) 10 m/s.