If vec a,vec b,vec c are three non–coplanar vectors with then

We have $\vec a+\vec b+\vec c=m\vec d=m[n\vec a-\vec b-\vec c]$

Since, $\vec a,\vec b,\vec c$ are non–coplanar,

1 – mn = 0, 1 + m = 0, ⇒ m = –1, n = –1.

Hence, $\vec a+\vec b+\vec c=-\vec d$     $⇒\vec a+\vec b+\vec c+\vec d=0$

Hence (D) is the correct answer.