A force F =α i +3 j +6 k is acting at a point r =2 i -6 j -12 k. The value of α for which angular momentum about origin is conserved is:

A force $\displaystyle \vec{F}=\alpha \hat{i}+3 \hat{j}+6 \hat{k}$ is acting at a point $\displaystyle \vec{r}=2 \hat{i}-6 \hat{j}-12 \hat{k}$. The value of $\displaystyle \alpha$ for which angular momentum about origin is conserved is :