Which underlined phrase is a prepositional phrase?

Test-takers were asked not to use their notes, books, or calculators.

Before passing out the test, the teacher asked students to close their books.

Neither of the tests this week had questions about the Pythagorean theorem.

The teacher and the students thought that the test took longer than expected.

Before passing out the test, the teacher asked students to close their books

Option 2: A Before passing out the test, the teacher asked students to close their books.

Before passing out the test is the prepositional phrase.

A prepositional phrase has the following basic characteristics: usually begins with a preposition and ends with a noun, pronoun or gerund, functions as a unified part of speech, doesn't have a verb nor a subject.

Before is a preposition

Passing out the test is a gerund, which means that it acts as a noun in the sentence, and at the same time is the "object" of the proposition.

The whole phrase is a unified part of speech and doesn't have any verb nor subject.