The phrase "a favorite topic of songwriters" is an interrupting phrase, a phrase that provides information but is not essential to the structure of the sentence. All interrupting phrases need to be set apart by commas; therefore, "Heartbreak, a favorite topic of songwriters," is the correct answer choice.

The phrase "unlike his colleagues" is an interrupting phrase, a phrase that provides information in the sentence, but is not a element key to its structure. All such phrases need to be set apart by commas; therefore, "The actor, unlike his colleagues, never performed" is the correct answer choice.

The phrase "a radical journal" is an appositive. It provides extra information in the sentence by renaming the noun "the newspaper," but remains apart from the sentence's structure. An appositive always needs to be set apart by commas when the removal of the its information would not change the meaning of the sentence. The only answer choice that does this is "The newspaper, a radical journal, often sought to foment rebellion."

The phrase "a highly controversial issue" is an interrupting phrase, or one that provides extra information while remaining apart from the structure of the sentence. That is, "a highly controversial issue" could be removed from the sentence and the sentence would still make sense. Interrupting phrases that don't provide necessary information need to be set apart from the rest of the sentence by commas. "School placement, a highly controversial issue, has been" is the only answer choice that does this, so it is the correct answer choice.

In the sentence, the phrase "unaware of the historical circumstances" is an interrupting phrase, or a phrase that provides information but is not a necessary component of the complete sentence. Such clauses always need to be set apart from the rest of the sentence by commas. Thus, "Many Americans, unaware of the historical circumstances," is the correct answer choice.

The phrase "the band's lead singer" is an appositive phrase; it renames the subject (here, "Francine") and provides additional information about the subject. In this case, the phrase "the band's lead singer" could be removed from the sentence without changing its meaning, so it needs to be surrounded by commas.

The phrase "as disputatious as ever" is a non-restrictive interrupting phrase, or a phrase that provides extra information and is not necessary to the meaning of the sentence. Any such phrase must be set apart from the rest of the sentence by commas. As such, "The boys in the band, as disputatious as ever," is the correct answer.

The word "Frequently" in the sentence is used as an adverb describing the actions of the students; therefore, it must be set apart from the rest of the sentence, and "students" in particular, to work as an introductory phrase. "Frequently, students" is the correct answer.

The phrase "day by day" is used in the sentence as an introductory phrase, modifying the meaning of the sentence. As such, it needs to be set off from the rest of the sentence. "Day by day, fewer" is the correct answer choice.

The phrase "when researching employment opportunites" operates in the sentence as an introductory conditional phrase, giving a necessary "if" to the sentence. Such a clause needs to be appropriately set apart from the rest of the sentence; therefore, "employment opportunities, college students" is the correct answer choice.