The border between the United States and Canada is the longest undefended border in the world, and many lawmakers are starting to argue that if parts of the border are not secured, the citizens of United States have faced an unknown threat.

(A) if parts of the border are not secured, the citizens of the United States have faced an unknown threat.
(B) the citizens of the United States will always face an unknown threat if they did not secure it.
(C) without securing it, the citizens of the United States will always face an unknown threat.
(D) always would the citizens of the United States face an unknown threat if it is not secured.
(E) if parts of the border are not secured, the citizens of the United States will always face an unknown threat.
_________________

If you liked my post, kindly give me a Kudos. Thanks.

The border between the United States and Canada is the longest undefended border in the world, and many lawmakers are starting to argue that if parts of the border are not secured, the citizens of United States have faced an unknown threat.

(A) if parts of the border are not secured, the citizens of the United States have faced an unknown threat. (In conditional sentence, usage of perfect tense is wrong)
(B) the citizens of the United States will always face an unknown threat if they did not secure it. (Usage of they is wrong. it may represent citizens)
(C) without securing it, the citizens of the United States will always face an unknown threat. (Usage of it is ambiguous)
(D) always would the citizens of the United States face an unknown threat if it is not secured. (Same as C)
(E) if parts of the border are not secured, the citizens of the United States will always face an unknown threat. (Correct)
_________________

Kindly hit kudos if my post helps!