Cross out any prepositional phrases. Underline the subject once and the verb or verb phrase twice. Label any direct object (D. O.) (Sit, Set) along the low wall past the small wooden bridge.

Prepositional Phrase - along the low wall

Subject - You

Verb - sit

D.O. - The low wall.

Explanation:

Prepositional phrases are characterized as the phrases that consist of a preposition followed by its object or complement and function primarily as an adjunct or modifier of the noun or verb to add detail to the sentence. While the subject is the one who performs the action in the sentence. Since the sentence is imperative(begins with a verb), the subject(implied) would be 'you.' Verb is the primary action performed by the subject i.e. 'sit' and the direct object is that which directly receives the action performed by the subject i.e. 'the low wall.'

the answer should be d) he was one of those young thousands who greeted the war with a certain amount of relief, welcoming the liberation from webs of tangled emotion" since it is the best option because it shows his desire to free himself. 

hope this answer you! have a great day.