The vector that must be added to the vector $\hat{i}-3 \hat{j}+2 \hat{k}$ and $3 \hat{i}+6 \hat{j}-7 \hat{k}$ so that the resultant vector is a unit vector along the $y$ -axis is
(a) $4 \hat{i}|2 \hat{j}| 5 \hat{k}$
(b) $\quad 4 \hat{i} \quad 2 \hat{j} \mid 5 \hat{k}$
(c) $3 \hat{i}|4 \hat{j}| 5 \hat{k}$
(d) null vector