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."

The phrase "Unlike his counterparts in the army" is an introductory phrase that conditions the action of the sentence. All such phrases must be set apart from the rest of the sentence by a comma. "Unlike his counterparts in the army, the sailor" is the correct answer choice.

The phrase "For eight straight weeks" is an introductory phrase, or one that conditions the meaning of the sentence but is not a crucial element of its structure. All such phrases need to be set off from the rest of the sentence by a comma. The only answer choice that appropriately does this is "For eight straight weeks, the area saw heavy rains that contributed to the massive flooding."

The phrase "For the absolute diehard fans" is an introductory phrase, a phrase that conditions the action of the sentence, but is not a necessary part of its structure. All introductory phrases must be set apart from the rest of the sentence by a comma. The only answer choice that does this is "For the absolute diehard fans, the loss was a crushing blow to their spirit."

The phrase "For the rest of his career" is an introductory phrase, a phrase that conditions the meaning of the rest of the sentence but is not crucial to the sentence's structure. All introductory phrases must be set apart from the rest of the sentence by a comma. Therefore, the correct answer choice is "For the rest of his career, he regretted never completing that specific sale."