A prepositional phrase starts with a preposition; it also includes the object of the preposition and any modifiers of the object.
C starts with the <em>preposition</em> to and includes the <em>object </em>of the preposition structure and two <em>modifiers</em> of the object your, sentence.
The prepositional phrase in this sentence is c. "to your sentence structure", because it is postmodifying an infinitive phrase (to bring complexity), which has the same value, in this case, as a noun phrase. Besides it is composed by a preposition (to) and a noun phrase (your sentence structure).
Option a. is a verb phrase as well as option b. In the same way, letter d. is not a prepositional phrase as it is an adjective phrase used in the comparative form.
