The last one is the answer
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.
The first sentence, ‘My cat’s favorite pastime seems to be sleeping’.
A gerund is when a vets is used as a noun.
Future tip: gerunds most always end in ‘ing’.
Hope this helps.
C as it has a better view on the situation