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.
