The correct answer is option 'A', which is: "The sun set behind the mountains."

Here's why option 'A' is the correct word order:

1. Subject-Verb-Object word order:
In English, the typical word order in a sentence is Subject-Verb-Object (S-V-O). This means that the subject (who or what the sentence is about) usually comes before the verb (the action or state of being), and the object (the receiver of the action) comes after the verb. In this sentence, "The sun" is the subject, "set" is the verb, and "behind the mountains" is a prepositional phrase acting as an adverbial phrase. So the correct word order follows the S-V-O pattern.

2. Placement of adverbial phrases:
Adverbial phrases provide more information about the verb, such as when, where, or how the action happened. In this sentence, "behind the mountains" is an adverbial phrase that tells us where the sun set. Adverbial phrases usually come after the verb or at the beginning or end of the sentence. In this case, it follows the verb "set" and comes at the end of the sentence, maintaining the correct word order.

3. Proper placement of prepositional phrases:
"Behind the mountains" is a prepositional phrase that consists of the preposition "behind" and its object "the mountains." Prepositional phrases usually provide information about location, time, or direction. In English, prepositional phrases typically come after the verb or at the beginning or end of the sentence. In this sentence, the prepositional phrase is correctly placed after the verb "set" and at the end of the sentence.

4. Coherence and clarity:
Option 'A' provides a clear and coherent sentence structure. It is easy to understand that the sun is the subject, setting is the action, and the mountains are the location. The word order in option 'A' ensures that the sentence is grammatically correct, logical, and conveys the intended meaning effectively.

Overall, option 'A' is the correct word order because it follows the Subject-Verb-Object pattern, correctly places the adverbial phrase, and maintains coherence and clarity in the sentence structure.