Heat can never flow from
a. hot to cold
b. cold to hot
c. liquid to solid
d. solid to liquid

The correct sentence is:
Heat can never flow from b. cold to hot.

In fact, heat always flows from hot to cold objects. The reason is the following: in an object with higher temperature, the particles move/vibrate faster than the particles in an object with lower temperature. When the two objects are in contact, therefore, the particles of the hotter object have more energy than the particles of the colder object, so the particles of the hotter object transfer more energy to the particles of the colder object, on average. As a result, heat always flows from hot objects to cold objects.

The earths atmosphere is made up of nitrogen and oxygensolids turning into gases is called sublimation.