Seismic P-wave speed changes with depth as the Earth's material properties change, and comparing typical values layer by layer answers this question directly.
- Upper mantle: P-waves travel at roughly 8 to 10 km/s here, well short of 13 km/s.
- Outer core: at the core mantle boundary the rock gives way to liquid iron alloy, and the P-wave speed falls sharply to around 8 km/s, only climbing back up to about 10 km/s by the base of this layer. It never reaches 13 km/s because the liquid state limits how fast a P-wave can travel here compared to the solid mantle just above it.
- Lower mantle: pressure keeps building with depth through this solid layer, and P-wave speed rises steadily, reaching about 13.5 to 13.7 km/s just before the boundary with the core. This is the only layer among the four where the speed genuinely goes past 13 km/s.
- Inner core: even though this layer is solid again, its P-wave speed is only about 11 km/s, still below the 13 km/s mark.
Only the lower mantle shows a P-wave speed above 13 km/s, so option (C) is correct.
Let's summarize:
- P-wave speed rises through the solid mantle with depth and peaks near the base of the lower mantle.
- It drops sharply on crossing into the liquid outer core and only partly recovers by the inner core.
So the lower mantle is the layer where P-wave velocity exceeds 13 km/s.