Read the sentence below and answer the question that follows.

The citizen was told that if he crossed the boundary into the other king’s territory, he would be arrested.

The underlined word in the sentence above can best be defined as a .
A.
high mountain that is challenging to cross
B.
group of soldiers hired to protect the king when he travels
C.
line that surrounds territory controlled by a person in power
D.
large structure in the middle of a kingdom used to hold armor