It should be what the other person said all
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.
"Essay on MY best teacher" We do not know your teachers and you did not provide any valid information on them, nobody can help without more data and you should be doing this alone because we dont know them.
Answer:
East
Explanation:
If a Mother had four sons named North, South, and West, the first sons name is most likely East. This is because North, South, East, and West are the different directions you might see on a compase. Since she used 3 out of 4 of the names, it is likely that the other sons name is East.