"The music sharing service considered a threat to copyright protection" is a phrase modifying the subject noun "Napster." It is an appositive phrase and should, therefore, be offset by two commas. Appositives define or rename the subject of the sentence, in this case Napster (the subject) is being re-characterized as "the music sharing service considered a threat to copyright protection." The appositive must be offset with commas because it is outside of the fundamental grammatical structure of the sentence.

In the underlined portion of the sentence, "in fact" is an interrupting phrase, a phrase that adds extra meaning to the sentence, but is not a part of the sentence's main structure. Any interrupting phrase must be set apart from the rest of the sentence by commas. The only answer choice that makes this correction is "that convinced him his father was, in fact, guilty."

The phrase "which was a process fraught with problems" is an interrupting phrase, one that adds meaning to the sentence, but is not a part of the main structure of the sentence. All interrupting phrases must be set apart from the rest of the sentence by commas. "Working together, which was a process fraught with problems, the two" is the only answer choice which appropriately sets off the interrupting phrase.

The phrase "a massive geothermal spot in the middle of the landmass" functions in this sentence as an interrupting phrase, one which conditions the meaning of the sentence, but sits outside the core structure of the sentence. Any interrupting phrase must be set apart from the rest of the sentence by commas. "The volcano, a massive geothermal spot in the middle of the landmass, threatened" is the only answer choice which correctly deploys commas around the phrase.

The use of dashes to set off the parenthetical phrase explaining what the least possible world is (rather than simply renaming it, as an appositive phrase set off by commas would do) is the best solution for this sentence.

The parenthetical interrupting phrase "or what's left of it, at any rate" is associated with the phrase "In the Oxford Ordinatio," and should come before the comma that separates that phrase from the rest of the sentence. No other punctuation is needed to set off the parenthetical phrase from the phrase it is associated with, nor should this phrase be needlessly split into further parenthetical phrases.

In formal written English, em-dashes (—) or commas are used to set off interrupting phrases. Commas would also be correct if used to set apart the phrase "ever the optimist," but that option is not provided as an answer choice, so the option that uses em-dashes to set apart the phrase is the correct answer.

The relative clause beginning with "who served..." modifies William McKinley. This modifying phrase should be offset by two commas. The verb "was" is the predicate of the sentence - if you remove the modifying clause, you get "William McKinley was re-elected." This is correct, over "William McKinley and he was re-elected." Interrupting phrases should be able to be removed, and still leave a grammatically correct sentence behind.