What process provides the source or generates the thermal heat that leads to hotspots and melts the earth's crust?

hint: this is a review of he#1 & he#2 consider the definition of a hotspot. it is widely believed that hotspots form where the inner core makes contact with the mantle or lowermost asthenosphere.

select one:

a. radioactivity in the core

b. conduction in the lithosphere

c. convection in the asthenosphere

d. radiation from the sun