Which of the following sentences uses parallel structure?

a. it is important to exercise, stretching before running, and go to the gym.
b. my three favorite activities are writing, reading, and to go to the library.
c. there is nothing she likes better than to build, constructing, and inventing.
d. when you meet someone, you should be friendly, introduce yourself, and smile.

D. parallel structure just means that all of your verbs in the comma series line up in regards to tenses, here. in choice D, it's "BE friendly," "INTRODUCE yourself," and "SMILE." these are all present tense verbs.

the rest of the choices don't match--"exercise" and "stretching" are two different verb forms in choice A, "to go to the library" is the odd one out in choice B, ane "to build" doesn't match with the "-ing" verbs in choice C.