If unit vector $a=a_1\widehat{i}+a_2\widehat{j}+a_3\widehat{k}$, the possible values of $a_1,a_2$ and $a_3$ are

a,b,d

The magnitude of unit vector is 1.

$\therefore \left|\text{a}\right|\text{=1\hspace{0.17em}or\hspace{0.17em}}\sqrt{{\text{a}}_{\text{1}}^{\text{2}}{\text{+a}}_{\text{2}}^{\text{2}}{\text{+a}}_{\text{3}}^{\text{3}}}\text{=1}$

This equation is satisfied by options (a) , (b) (d), Hence , option

a,b and d are correct.