Earth's inner core is roughly as hot as...

Answer: The surface of the Sun

The surface of the Sun ✓ — Correct! Earth's inner core sits at about 5,400°C — within a hundred degrees of the Sun's visible surface (~5,500°C). A Moon-sized sphere of solid iron, buried 5,100 km beneath your feet, is nearly as hot as a star's surface.

A steel-melting blast furnace — Not quite. Industrial blast furnaces peak around 1,500°C — enough to melt steel at the surface, but only about a quarter of the inner core's temperature. The core is roughly 3.5× hotter than the hottest steel-making equipment.

The channel of a lightning bolt — Not quite. A lightning channel briefly reaches ~30,000°C — about 5× hotter than the inner core. But that heat exists for milliseconds in a channel millimeters wide. The core is a Moon-sized solid iron sphere held at 5,400°C for billions of years.