The word "unquestionably" is used in this sentence as an introductory phrase, giving a condition for the rest of the sentence (namely that the statement can not be questioned). An introductory phrase must always be set apart from the rest of the sentence by a comma. "Unquestionably, the most difficult thing" is therefore the correct answer choice.

The phrase "More than anyone else he had encountered" is an introductory clause, a clause that conditions the information provided in the main body of the sentence. All such clauses need to be set off from the rest of the sentence by a comma, making "More than anyone else he had encountered, the preacher was convinced of the rightness of his cause."

The phrase "Each year" in the sentence is an introductory phrase, a phrase that conditions the main part of the sentence but is not a necessary part of its structure. All introductory phrases must be set apart by a comma. As this is the only issue with the sentence as it is written, the correct answer choice is "Each year, thousands of animals."

The phrase "Under great stress" is an introductory phrase, one that conditions the meaning of the sentence but is separate from the main sentence structure. All such phrases need to be set apart from the rest of the sentence by a comma. The only answer choice to appropriately do this is "Under great stress, the president of the company."

The issue with the sentence is that its single word introductory clause, "Incidentally," is not appropriately set off from the rest of the sentence with a comma. The only answer choice that does that is "Incidentally, the boy was never at the site of the crime he was accused of committing." All other answer choices are grammatically incorrect.

The phrase "Unlike his classmates" functions as an introductory phrase, one that conditions the meaning of the rest of the sentence. All introductory phrases must be set apart from the rest of the sentence by a comma; therefore, "Unlike his classmates, the boy" is the correct answer choice.

The phrase "Keeping his promises" is an introductory phrase that conditions the action of the sentence, but is not crucial to the structure of the sentence. All such phrases need to be set apart by a comma from the main portion of the sentence. The only answer choice that does this is "Keeping his promises, the father," which is the correct answer.

The phrase "Per the guidelines" is an introductory phrase, a clause that conditions the meaning of the sentence, but is not part of the construction of the sentence. All such clauses must be separated by a comma from the rest of the sentence, making "Per the guidelines, everyone" the correct answer choice.

The phrase "More than most singers" is an introductory phrase that explains the conditions of the sentence's body. All introductory phrases need to be set off by a comma, and no other spots in the sentence need a comma. "More than most singers her life story has always been read into her song lyrics by fans," is the correct answer choice.

The phrase "Just like his sister" is an introductory phrase, or a phrase that conditions the meaning of the sentence without being a crucial part of its structure. All introductory phrases need to be set apart from the rest of the sentence by a comma. Therefore, the correct answer choice is "Just like his sister, he was."