The phrase "capable of playing a dozen instruments" is an interrupting phrase, one that adds extra information to a sentence, but sits outside the main structure of the sentence. Any interrupting phrase must be set apart from the rest of the sentence by commas. "The musician, capable of playing a dozen instruments, needed" is the only answer choice that correctly sets off the interrupting phrase.

The phrase "a strong and powerful man" is an appositive, a type of interrupting phrase that adds extra information to the sentence, but remains outside the main structure of the sentence. Any interrupting phrase must be set apart by commas from the rest of the sentence, making the correct answer choice "The contractor, a strong and powerful man, was able to lift two bags of cement mix at once."

The phrase "a staunch disciplinarian" is an interrupting phrase, one that adds extra information to the sentence, but remains outside the main structure of the sentence. An interrupting phrase needs to be set apart from the rest of the sentence by commas. The only answer choice that properly sets apart the interrupting phrase is "The new coach, a staunch disciplinarian, made the team work harder than they ever had."

The phrase "a well respected lawyer" is an interrupting phrase, one that adds information to a sentence but remains apart from its main structure. Any interrupting phrase must be set apart from the rest of the sentence by commas. The only answer choice to correctly do this is "father, a well respected lawyer, was."

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.