Hi!
Option A is correct.
In cooking, convection is the phenomena that causes molecules with a higher energy at the bottom of a container to travel upwards, and the molecules at the top with a lower energy to sink to the bottom and replace them.
Mechanical convection describes this phenomena as a result of the application of an external force.
This phenomena is restricted to substances that are in liquid, or gaseous state, as molecules of a substance in solid state have a very restricted movement.
Hence, options B, C and D are incorrect.
Hope this helps!