Six charges are placed at the vertices of a regular hexagon as shown in the figure. The electric field on the line passing through point O and perpendicular to the plane of the figure at a distance of $x\left(>>a\right)$  from O is

This is basically a problem of finding the electric field due to three dipoles. The dipole moment of each dipole is $P=Q\left(2a\right)$
 Electric field due to each dipole will be$E=\frac{Kp}{x^3}$
$E_{net}=E+2E\cos {60}^0$

$=2E=2\times \frac{1}{4\pi {\epsilon }_o}\times \frac{2Qa}{x^3}$$=\frac{Qa}{4\pi {\epsilon }_o{x}^3}$
The direction of electric field due to each dipole is as shown above in the figure. 
Hence, the correct answer is option (a).