Which sentence correctly uses commas to separate phrases? the cooking class will teach us how to, grill fish, bake a pie, and make cream. the math test will include one word problem, one graph and ten equations. i spend my free time reading books, watching tv, and playing video games. staying fit involves eating healthy foods, staying active and, sleeping well.

The correct answer is C, "I spend my free time reading books, watching TV, and playing video games."  

A comma is needed to separate items in a list. It does not need to go before the list, but should be included before and (although there are people who believe this comma is not necessary). Also remember that all of items in your list must be in the same tense (present, past, or present progressive).

faulty parallelism is a grammar construction where there is no balance in the parts of a sentence they have a similar meaning but the grammar structure is not equivalent.

the correct version of this faulty parallelism in the sentence is:

jefferson wrote the declaration of independence, and he was the nation's third president.

the problem with the first version is that the sentence:

"jefferson was both the writer of the declaration of independence"

presents jefferson as both a subject and an agent for the verb "write" because if he was "the writer" then the declaration of independence was written by him, besides that fact, the sentence presents a dependent clause.

while the second sentence:

he was the nation's third president.

presents a basic subject+verb+object structure

so, to give a proper balance to the structure, or better said, to give a parallel structure, we must write a basic s+v+o structure in both sentences:

jefferson wrote the declaration of independence, and he was the nation third president.