The correct answer is D. Cannot stand as a sentence.
Subordinate clauses convey no meaning when left alone and must always be used in combination with independent clauses so as to create a complex sentence that conveys meaning.
d, it’s giving the most info without saying too much
Border marks where a country is : a country is encompassed by borders. This is similar to ocean and shores: shores mark where the ocean is: an ocean is encompassed by the shores.
The answer is d.
