An example of natural convection would be
a. placing an ice cube in your hand.
b. the movement of the mantle inside earth.
c. getting warmed by the sun.

the movement of the mantle inside the earth.

The natural convection is a process, in which the fluid movement occurs from heat transfer. When a fluid is warmed or chilled, its density changes and the buoyancy effect create a natural flow in the affected region, which makes itself the rise of hotter fluid and the fall of cooler fluid: hence, energy transfer from hotter area to colder area and such process is repeated as long as the temperature variation in fluid exist.

Mantle convection is the quiet creeping motion of Earth's solid silicate mantle produced by convection currents conveying heat of the inside to the planet's surface.

prism b but im not for sure

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.