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."

The phrase "More than any other sport" is an introductory phrase, a phrase that conditions the meaning of the sentence, but is separate from the structure of the sentence. All introductory phrases must be set off from the rest of the sentence by a comma, making the correct answer "More than any other sport, soccer requires immense amounts of stamina from its players."

The phrase "unlike his predecessor" is an introductory phrase, one that conditions the meaning of the sentence, but remains apart from the sentence's structure. All introductory phrases must be set off from the rest of the sentence by a comma. The only answer choice that appropriately does this is "Unlike his predecessor, the new CEO created a climate of openness and camaraderie."

The phrase "To placate his father" is an introductory phrase, one that conditions and explains the main action of the sentence. All introductory phrases need to be set apart from the rest of the sentence by a comma. The only answer choice that does this is "To placate his father, the teenager signed up to take Latin courses in college."

The phrase "Having been running around the same area" is an introductory phrase, one that conditions the action of the sentence. All introductory phrases need to be set off from the rest of the sentence by a comma. The only answer choice that does this is "Having been running around the same area, the boy."

The phrase "To be better prepared" is an introductory phrase, one that conditions the main action of the sentence, but remains apart from the sentence's main structure. Any introductory phrase needs to be set apart from the rest of the sentence by a comma. Therefore, the correct answer choice is "To be better prepared, the debate club."