Answer:
Well, I assume that you are referring to the SECOND sentence. The only error is the missing obligatory comma after the coordinating conjunction "so."
Explanation:
Germany had to fight to many upfront which lead to it's defeat
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.
