Thermal (contact) metamorphism occurs
a. only where gneiss is in contact with schist
b. only at the surface, where rock is in contact with the atmosphere
c. as a consequence of the sinking of a broad region to great depth
d. in areas surrounding igneous intrusions

Option (D)

Explanation:

The contact metamorphism is a type of metamorphism which usually occurs around an intrusive igneous body. The temperature of the adjacent rock increases due to the intrusion of hot molten magma. The area that surrounds this intrusive body forms a ring which is typically known as the metamorphic aureole. The rocks that are commonly formed due to this type of metamorphism is hornfels. This rocks are mostly fine-grained and they do not indicate if the deformation is strong or weak.

Thus, this type of metamorphism generally takes place in the areas surrounding an igneous intrusion.

Hence, the correct answer is option (D).

i'm sure the answer you are looking for would be

a. an ice age

hope this contact me if this didn't or you need with more answers : )