If $a=i+2 j+k, b=i-j+k$ and
$\mathbf{c}=\mathbf{i}+\mathbf{j}-\mathbf{k}$, A vector in the plane of a and $\mathrm{b}$ whose projection on $\mathbf{c}$ is $1 / \sqrt{3}$, is
(a) $4 \mathrm{i}-\mathbf{j}+4 \mathbf{k}$
(b) $3 \mathbf{i}+\mathbf{j}-3 \mathbf{k}$
(c) $2 \mathbf{i}+\mathbf{j}-2 \mathbf{k}$
(d) $4 \mathbf{i}+\mathbf{j}-4 \mathbf{k}$