Hi Christine,
The leading edge means the front of the plate, and the trailing edge means the back of the plate.
Imagine this arrow is a plate.
The plate is moving in the same direction as the arrow. The leading edge is the pointy part of the arrow, and the trailing edge is the flat part of the arrow.
So let's imagine both plates now as these two arrows.
They are moving in the same direction. According to the passage, right around line 30, we see that the back plate is moving faster, and the leading edge of the back plate slides under the trailing edge of the front plate. Answer choice (C) is close, but it says that the trailing edge of the first plate goes under the leading edge of the second. That doesn't match the description in the passage, so it is incorrect.
Hope that helps!