Answer:
C.
Step-by-step explanation:
The correct option to fill this statement is C.
In the given statement, the writer is referring to the ocean where Mark went for swimming. To describe the large area of ocean the correct word is expanse. The term expanse can be defined as a wide area of something that seems endless. So, Mark was amazed to see the great expanse of the seemingly never ending water.
In the second statement, Mark felt small in comparison with the size of the ocean. So, the correct word here would be vastness. The term vastness is used to describe great size of something.
So, option C is correct.