Question

What is the prepositional phrase in the sentence? This summer, we will host an exchange student from Ecuador. A. This summer B. we will host C. an exchange student D. from Ecuador.

Answer 1

D. A preposition is where the subject is.

Answer 2

Short Answer D
Remark
A prepositional phrase begins with a preposition. (from is a preposition). There isn't another one in this sentence.

A
this summer "this" is a determiner. It defines which summer (this one). It also answers when. So This summer is an adverb modifying host. This example is hard to break down. A is not right.

B
We is the subject of the sentence
will host is the verb. Neither are prepositions. 
B is not right.

C
An exchange student is the object of will host.

D
From Ecuador is the answer you want. it modifies student (what kind of student. Where's he from.