Maybe you could be a good writer—maybe even good enough to write a book or articles in a newspaper—but you might not know it until you write a paper for your English class. Maybe you could be an innovator or an inventor—maybe even good enough to come up with the next iPhone or a new medicine or vaccine—but you might not know it until you do a project for your science class.

In this section of the speech, the purpose of the parallel structure is to help Obama:
A. emphasize his belief in the importance of being able to write well.
B. explain his thoughts on the best classes to take for a good education.
C. prove his point by spelling out the options available with an education.
D. strengthen his argument that more scientists are needed in the workforce.

The purpose of parallel structure is to help Obama:

C. prove his point by spelling out the options available with an education.

Explanation:

From this passage, we can safely understand that President Obama's purpose is to convince his audience of the importance of education. He uses parallel structure -  the repetition of a certain grammatical structure inside a sentence - to prove his point by spelling out the options people have with an education. The structure he repeats is:

- Maybe you could be a... - maybe even good enough to... - but you might nor know it until you...

With that structure, he offers examples of the professional level of excellence people can reach in life by exploring their talents. He also makes it clear that school will help those people find and explore such talents. It is education that will open those students' eyes to what they can do and achieve in life.

Option C is the correct answer,prove his point by spelling out the options available with an education.

Explanation:

Option A is wrong because the parallel structure is not just about the being able to write well.

Option D is wrong for the same reason as above since writing well was also mentioned in the parallel structure not just being a scientist.

Option  B is also wrong because the speaker was not talking about attending classes,the points highlighted were broader than that.

Option C is right  as the speaker explored diverse fields of human endeavors to drive home the point that there are different options available in educational career path.

i think the answer would mist likely be an adjective clause, because unhappy is an adjective.