The time the particles will take to meet each other is t = 2a/v as the particle needs to cover a distance of a along the hypotenuse of the triangle to reach the other particle.
Option B is correct
We have that Time (t) = Distance (d) / Relative Velocity (v)
t = a / v.
This applies to each pair of particles moving towards each other. Since there are six particles, there are three pairs moving towards each other
Their paths in reality do not affect each other, and they will all collide at the center of the hexagon after the same time
(t = a / v).
Therefore, the overall time for all particles to meet at the center is also t = a / v and our answer becomes t = 2a/v.