Which sentence uses a infinitive phrase as a modifier?
a) do you want to pass my class with the least amount of effort possible?
b) on the camping trip, all of the women gathered firewood for the first night.
c) the best way to cook a grilled cheese is with a strip of bacon in the middle.
d) the people who designed the scavenger hunt made it more difficult than necessary.

The sentence that uses a infinitive phrase as a modifier is option C. The best way to cook a grilled cheese is with a strip of bacon in the middle.

An infinitive phrase is the infinitive form of a verb plus any complements and modifiers.  The infinitive phrase can be a subject, an adjective, an adverb, a direct object, or a predicate nominative in a sentence.  In the case of and adjective it tends to modify a noun placed before.

postards have pictures on them, these pictures are things that may never be seen again. the pictures are a part of our history that shows things. because of this many collectors would pay a lot of money for them.

i read on google they "offer a glimpse into the past"

hope this : )

i think the answer would mist likely be an adjective clause, because unhappy is an adjective.