"The GMAT builds character" and "it is full of challenging sentences" are independent clauses. They are related, but independent; therefore, the best way of separating them within the same sentence is with a semicolon rather than a comma. 

The comma inserted in the underlined portion of the sentence between "margins" and "over" is unnecessary. The full phrase should run continuously without interruption, as it is all one thought. The only answer choice with appropriate punctuation is "in profit margins over the final quarter of the year."

The phrase "All his life" is an example of a 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 "All his life, the boy wanted."

The phrase "a convertible" is a paranthetical 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 "wanted, a convertible, for her 30th birthday."

The phrase "When he picked up his kids from school" is an example of a 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 "When he picked up his kids from school, he surprised them."

The words "big" and "blue" are coordinate adjectives: adjectives that modify the same noun separately, but are not separated with a conjunction. Coordinate adjectives need to be separated with a comma. The correct answer choice is "They wanted a big, blue banner."

The phrase "which is one of the largest buildings in the world"  is an example of a relative clause, which gives more information about its antecedent: the Willis Tower. 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 Willis Tower, which is one of the largest buildings in the world, was built."

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 "because he is honest, and he likes to help others."

The phrase "Jogging around his neighborhood" is an example of a 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 "Jogging around his neighborhood, he saw something strange."

The phrase "listening to music" 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 girl's favorite pastime, listening to music, eventually lead to her career as a professional singer."