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.
Grover let Percy's mom "die" and was supposed to give Percy a safe passage to Camp Half Blood.
D - social power the best answer