Answer:
C. One protects other.
Explanation:
First, you should eliminate what isn't reasonable.
D says "both are used in urban environments" this answer choice doesn't seem that reliable, and it doesn't relate AT ALL to the analogy, because castles and helmets aren't even of modern time, and more medieval times.
B. says "one purchases the other" this doesn't make any sense, because a castle doesn't buy a moat, and a head doesn't buy a helmet, though you can buy them, they don't buy each other.
Now you are left with A and C
A says covers the other, while C says protects the other. If you do not know, a moat of a castle is actually the bridge in front of a castle that can be considered the passage of entry and the exit as well. This can be considered as a form of protection to keep out any enemies. So, the actual answer is C.
