Which statement about tectonic plates is true?

• A. A weather phenomenon
• B. An boundary where two plates meet
• C. A molten layer in the mantle
• D. A rigid segment of the Earth's crust which can float on the heavier, semi-molten rock below

Answer: (D)

Think about the plates as rigid pieces of the outer shell of the Earth that can move because they ride on a softer, partly molten layer beneath. A tectonic plate is exactly that: a rigid chunk of the crust and upper mantle that floats on the heavier, semi-molten rock of the mantle below. This floating and moving is what lets plates shift, collide, slide past each other, or pull apart over geological time.

The other statements don’t describe a plate itself: one points to a weather phenomenon, which is unrelated; another describes a boundary where plates meet (that’s about interactions, not what a plate is); and the last points to the molten layer in the mantle (the asthenosphere) rather than a plate.