Which part of Earth is solid and extremely hot? A. inner core B. outer core C. mantle D. crust

The right answer is indeed A. inner core.

Explanation:

The Earth's structure can be divided into four parts: crust, mantle, outer core, and inner core. The crust is the outermost layer, having an average depth of 30km when it comes to continental crusts, and 5km for oceanic crusts. The mantle lies beneath the crust and presents a depth of 2,900km. It is constituted of hot rock, between 900 and 2,200 degrees Celsius. Next we have the outer core, consisting or liquid metal - nickel and iron - due to the extreme temperatures ranging from 2,200 and 5,000 degrees Celsius. Finally, beneath the outer core we have the inner core. Even though the temperature is extremely high - 5,000 degrees Celsius -, the inner core is solid due to the high pressure it is under, which is 3 million times higher than on the crust.