Final answer:
Triplet A ages the most, Triplet B ages less, and Triplet C ages the least.
Step-by-step explanation:
The question asks about the age of each triplet during a space travel scenario involving time dilation due to relativistic speeds.
Triplet A stays on Earth, while Triplet B travels at a speed of 4c/5 to a planet L away and then returns. Triplet C travels to the same planet at a speed of 3c/4 and returns at the necessary speed to arrive back exactly when B does.
According to the theory of relativity, time dilation occurs at high speeds, meaning that time moves slower for objects in motion.
As a result, Triplet B, who ages less, will be the youngest. Triplet C will age more than B but less than A, so C will be the second youngest. Triplet A, who remains on Earth, ages the most and will be the oldest.