"In the Middle Ages" is an introductory phrase, a phrase that conditions the meaning of the sentence, but remains outside its main structure. Any introductory phrase must be set apart by a comma from the rest of the sentence. "In the Middle Ages, few peasants were ever able" is the only answer choice to appropriately use such a comma.

"Unlike his brothers" is an introductory phrase, one that conditions the meaning of the sentence, but remains outside its main structure. Any introductory phrase must be set apart from the rest of the sentence by a comma. "Unlike his brothers, the little boy hated" is the only answer choice that correctly uses a comma.

"Under the new treaty" is an introductory phrase, one that condistions the action of the sentence, but remains outside of the sentence's main structure. An introductory phrase must always be set off by a comma. The only answer choice that correctly sets off the phrase is "Under the new treaty, neither side could put defensive works along their border."

"In the previous case" is an introductory phrase, one that conditions the meaning of the sentence, but remains separate from the main body of the sentence. Any interrupting phrase must be set off from the main portion of the sentence by a comma. The only answer choice that correctly sets off the phrase is "In the previous case, the exact same weapon was used."

"Under the teacher's guidance" is an introductory phrase, one that conditions the action of the sentence, but is apart from its main structure. Any introductory phrase needs to be set apart fom the main body of the sentence by a comma. The only answer choice that correctly uses a comma to separate the phrase is "Under the teacher's guidance, the students' reading ability improved."

"Unlike his family" is an introductory phrase, one that conditions the action of the sentence, but remains apart from its main body. Any introductory phrase must be set apart from the rest of the sentence by a comma. The only answer choice that correctly uses the comma is "Unlike his family, the man became deeply religious as he got older."

"Just like his father" is an introductory phrase, one that conditions the action of the sentence, but remains outside its main structure. Any introductory phrase must be set apart from the body of the sentence by a comma. The only answer choice that appropriately uses a comma in this way is "Just like his father, the little boy had flaming red hair."

"For the new model" is an introductory phrase, one that conditions the action of the sentence, but remains outside of its main structure. Any introductory phrase must be set apart from the rest of the sentence by a comma. The only answer choice which properly uses the comma is "For the new model, everything about the car was changed."

The phrase "For the new team" is an introductory phrase, one that conditions the meaning of the sentence but remains outside its main structure. All introductory phrases need to be set apart from the sentence by a comma. The only answer choice to correctly do this is "For the new team, the coach intentionally sought out different kinds of players."

It may take you a moment to figure out that "the raccoon" is not actually the subject of the sentence and "this mammal" is since the nouns appear right next to each other. The phrase "Unlike its distant cousin the raccoon" is an introductory phrase, giving a condition in which to understand the main body of the sentence. All introductory phrases need to be set apart from the rest of the sentence by a comma. Therefore, the correct answer choice is "Unlike its distant cousin the raccoon, this mammal."