Vatican city is considered an enclave because it is
a.
completely surrounded by rome, italy
b.
geographically separated from the mainland of italy
c.
a self-governing region within another state
d.
occupied by the roman catholic church

Enclave means a small territory sorrounded by a bigger one and whose inhabitants are culturally different.

So the answer would be A.
B isn't right, because it is not geographically separated. 
C, it is indeed a self-governing region within another state but it doesn't mean that the culture are different.
D. it is indeed, but in the definition of Enclave, it isn't fully relevant, because we want to know if one territory in inside another who is culturally different.

Can you give me little more information

It is the doctrine that the us had the right and duty to expand throughout north america. it just means they believed they had the right to expand westward and they did! they stated with the 13 colonies and then they kept moving gaining different territories throughout the years