What best defines a tectonic plate?

• A. A molten layer in the mantle
• B. A rigid segment of the Earth's crust which can float on the heavier, semi-molten rock below
• C. A boundary where plates intersect but do not move
• D. A type of rock

Answer: (B)

Tectonic plates are large, rigid pieces that make up parts of the Earth's outer shell and move as a unit over a weaker, partly molten layer beneath. This means a plate is best defined as a rigid segment of the crust (and the adjoining upper mantle) that can float on the heavier, semi-molten rock of the layer below. The other ideas miss what a plate actually is: a molten layer in the mantle isn’t a plate itself but the context that lets plates move; a boundary where plates intersect describes how they interact, not what a plate is; and a rock, while related, isn’t the whole structure—plates are chunks of crust and upper mantle. So the description that emphasizes a rigid segment floating on underlying, partly molten rock captures what a tectonic plate truly is.