i) The diagram shows that the 4 vectors enclose a quadrilateral, so the resultant is 0, therefore the sum of the vectors is 0.
ii) MN = (a + b/2) - (a/2)
MN = 1/2(a + b)
QP = (a + b + c/2) - (a + b + c - d/2)
= -1/2(c + d)
Now c + d = - (a + b) (using part i)
So MN = QP
Follow a similar argument to prove that NP = MQ, and then the proof is complete.