Answer: On the shore.
Explanation: a preposition is a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause. A prepositional phrase is a modifying phrase consisting of a preposition and its object. In the given sentence, the preposition is "on" and its object is "shore" so "on the shore is the prepositional phrase (also "The sailor" is the subject, "sang" is the verb and "songs" is the direct object of the sentence).
