As given vector u,v and w given may be not parallel but they may be antiparallel So,z≠au+bv+cw So first is incorrect. Also, if u=v+w v=u+w w=u+v Then, u+v+w=2(u+v+w) ⇒‌‌u+v+w=0≠z So, option (b) is incorrect. Similarly, option (c) is incorrect.