Which of the following sentences uses parallel structures?

a) it is important to exercise, stretching before running, and go to the gym.
b) my three favorite activities are writing, reading and going 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.

The sentence that uses parallel structures is B) My three favorite activities are writing, reading, and going to the library.
A parallel structure means that the same form of a verb is used in a sentence, and as you can see, here only gerunds are used - writing, reading, going - they all have -ing in the end.
The other sentences don't have a parallel structure because they don't use the same forms.