The phrase "which was necessary for John's job" is an example of a relative clause, which gives more information about its antecedent: the camera. Any such phrase that occurs in the middle of a sentence, often called an interrupting phrase, must be introduced and completed with a comma. The correct answer choice is "The camera, which was necessary for John's job, no longer worked."

In this sentence, two independent clauses are being joined by a coordinating conjunction ("for"). When this occurs, a comma must come before the coordinate conjunction. The correct answer choice is "he was trying to solve the problem, for his boss promised a bonus to the first person that could do it."

The phrase "who did not like how they were being treated"  is an example of a relative clause, which gives more information about its antecedent: workers. Any such phrase that occurs in the middle of a sentence must be introduced and completed with a comma. The correct answer choice is "The workers, who did not like how they were being treated, decided to go on strike."

Such phrases are often called "interrupting phrases." Interrupting phrases are used to provide information that or provide useful context about (usually) the subject of the sentence, but they are not grammatically necessary to the overall structure of the sentence. An interrupting phrase, properly enclosed by commas, could be excised from the sentence and the sentence would remain grammatically viable.

The phrase "During his vacation" is an example of an introductory phrase, explaining information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence. The correct answer choice is "During his vacation, Dave visited many countries."

In this sentence, two independent clauses are being joined by a coordinate conjunction. When this occurs, a comma must come before the coordinate conjunction. The correct answer choice is "New York is a major financial center, and many people flock there every year looking for work."

Note here that while a comma is required before "and" there is no need for any other commas. Placing a single comma in the middle of a clause is itself an error.

In this sentence, two independent clauses are being joined by a coordinate conjunction. When this occurs, a comma must come before the coordinate conjunction. Furthermore, the phrase "with this decision" that occurs in the second half of the sentence is a parenthetical expression: a phrase that explains more about a particular word or phrase in the sentence, but is not introduced by a relative pronoun. These phrases must be introduced and completed with a comma in order to separate them from the main clause of the sentence. The correct answer choice is "The company was opening a new branch office in Asia, and, with this decision, it was hoping."

The phrase "such as Jungle Region and Tree Fruit" is a parenthetical expression: a phrase that explains more about a particular word or phrase in the sentence, but is not introduced by a relative pronoun. These phrases must be introduced and completed with a comma in order to separate them from the main clause of the sentence. The correct answer choice is "Many modern companies, such as Jungle Region and Tree Fruit, seem to have been started in someone's garage."

The phrase "much time money and patience" is a list of qualities. All items in any sort of list/parallel words or phrases should be separated by commas. Additionally, a comma must come before the "and" because the action "money" and "patience" are not related in concept exclusively, so they should not be grouped together. The correct answer is "Organizing events takes much time, money, and patience."

In this sentence, two independent clauses are being joined by a coordinate conjunction. When this occurs, a comma must come before the coordinate conjunction. The correct answer choice is "The company's fans were very excited about its new product, but they were not enthused about the expected price."

In this sentence, two independent clauses are being joined by a coordinate conjunction. When this occurs, a comma must come before the coordinate conjunction. The correct answer choice is "Poetry readings are often quite boring, and many people, even poets, avoid them at all costs."