"His or her" would technically be the best fit, since it is acknowledging both genders, but it is very common to use "their" any most people wouldn't consider it wrong. <span />
3: prove his point by spelling out the options available with an education.
Obama talks about different professions in the speech, they aren't just limited to science or writing. This eliminates Options 1 and 2 as possibilities. Also he is not saying that you should take specific classes, but rather you should try and do your best in every class, because you never know which class may provide you with inspiration or the skills you need later in life.
Expository writing is meant to inform a reader about a certain topic. Because of this, D is the correct answer.
