Well the collection of large no. of particles having speed of each particle 'v' is practically not possible and if instantaneously if it is made so somehow than due to scattering (elastic and inelastic) and collision the situation will immedeatly chang.

 

Moreover, in the given case the average relative velocity of these particles will be zero as the distribution is random.

 

As far as the selection of pair of two particles for the purpose of magnitude of relative velocity between a a pair of particles is concerned then the solution or answer all depends on the pair of particles selected

This is because for the given pair the direction of velocities may be opposite or in the same direction or may subtend some angle.

 

So it is absurd to talk about relative velocities but ofcourse we can comment something on relative speed.

 

And if it is relative speed then definitely option is (b)