The underlined part of the sentence combines two independent clauses: "she realized that she had left her keys at home" and "she had rushed over too quickly." You can combine two independent clauses with a semicolon if they are closely related (such as if one clause causes the other, or if the second clause is explaining or clarifying the first). Only one answer choice correctly uses a semicolon without introducing other errors, and that answer choice is "she had left her keys at home; she had rushed over too quickly." The other options are incorrect because they create run-on sentences or use inconsistent verb tenses.

The answer choice "rocks; for this reason, the period" contains an appropriate use of a semicolon and is correct.

The original text "rocks, it is for this reason that the period" and the answer choice "rocks, for this reason the period" each create a run-on sentence, so neither of these answers is correct.

The answer choice "rocks, which for this reason, the period" contains a misplaced relative clause and is therefore incorrect.

The answer choice "rocks; and for this reason, the period" contains an incorrect use of a semicolon. Sentences can be joined by either a comma followed by a conjunction or by a semicolon, but not by a semicolon followed by a conjunction.

The part of the sentence that precedes the comma and the part of the sentence that comes after the comma are each independent clauses; they could stand as complete sentences by themselves. It is not grammatically correct to join two independent clauses together into a compound sentence by using just a comma; this is a grammatical error known as a "comma splice." Compound sentences may be formed using either a semicolon by itself or a comma followed by a conjunction. The only answer choice that correctly uses one of these methods to fix the sentence's comma splice is "The teacher was a stickler for order; she did not tolerate anyone acting up in her classroom."

The sentence is written as a compound sentence, joining two independent phrases into one sentence to show their relationship. All compound sentences, though, need to be joined either by a semicolon or a comma followed by a conjunction. The only answer choice which employs either of these options is "Television has been a controversial medium since its inception; critics were bemoaning its cultural status as early as the 1940s."

The sentence as it is written is a compound sentence, or two complete thoughts joined together to show their relationship. All compound sentences have to be joined by either a semicolon or a comma and a conjunction. The only answer choice that does this is "Young musicians often struggle with keeping the proper tempo in an ensemble; remedying this problem is one of the things that signals a musician's growing maturity."

The original sentence has a comma splice. Because the two clauses contrast each other, the conjunction but is most appropriate. One choice using but is wrong because the conjunction cannot be used with a semicolon. Another choice using but is wrong because the phrase "in cricket" is misplaced.

The sentence written above is a compound sentence, a sentence formed by two complete thoughts that are joined to show their relationship. The appropriate ways to join complete sentences is either with a semi-colon or a comma and a conjunction. The only answer choice that does this is "The politicians grew more and more nervous about the legislation passing with each day; former supporters were switching sides with regularity."

The sentence is a compound sentence, two complete thoughts that are joined in one sentence. All compound sentences need to have their two parts joined by either a semicolon or a comma and a conjunction; therefore, the correct answer is "Everyone was aware of the man's financial troubles, few knew just how deep in debt he truly was."

The sentence as it is written contains a comma splice, an error in which a comma is incorrectly used to combine two independent clauses. To fix the issue, one need only replace the offending comma with either a semicolon or a comma followed by a conjunction. The only answer choice that employs one of these options is "Many of the participants in the debate were completely unaware of the issues; they looked embarassed when they were asked questions about topics they had not reviewed."

Grammatically, it is best to avoid use of verbs ending in "-ing" where they are not necessary. The correct answer avoids this by using the phrase "owner who started." It is also the most efficient (and least wordy) option.