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.

The word "everyday" in the sentence acts as a conditional phrase, indicating how often people do what is described in the sentence. Such a phrase, must be set off by a comma from the rest of the sentence. Therefore, "Everyday, people" is the correct answer choice.

The underlined portion of the sentence contains a slight punctuation error. Speciifically, the word "Lately" is used as an introductory phrase, explaining the condition under which the action takes place, and such clauses need to be set off by a comma from the rest of the sentence. "Lately, families" is the correct answer.

The phrase "On this date" is an introductory clause, describing when the action of the sentence took place. An introductory clause must be set apart from the rest of the sentence by a comma. Therefore, "On this date, an unspeakable evil" is the correct answer choice.

The phrase "Unfamiliar to most listeners" is used in this sentence as an introductory phrase, something that explains the actions of the rest of the sentence. All such sentences must be set apart from the rest of the sentence by a comma. "Unfamiliar to most listeners, the band" is the correct answer.

The phrase "Working on a new book" is an introductory phrase that explains the condition upon the action of the sentence. All such phrases must be separated from the rest of the sentence by a comma. "Working on a new book, the author" is the correct answer choice.

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."