**Concept:** The question tests on the basics of Rates(Speed and Time)

**Solution:**

Let the length of the faster train = x m

Here we use Relative speed as there are 2 objects in motion simultaneously.

Since the objects are traveling in the same direction, the relative speed is the difference of speeds = (70 - 50) * 5/18 = 100/18 = 50/9 m /s

Time = D / RS

36 = x / (50/9)

x = 36 * 50/9 = 200 m **(option d)**