What is the dimension of n if force is given by the equation f=6pie rvn

F = 6Pi nrv

F = 6πnrv. 

Unit of force is Newton that is kgms⁻², r, radius is in m, v, velocity is in ms⁻¹.

6 has no unit, and same as π

F = 6πnrv

kgms⁻² = n*m*ms⁻¹

kgms⁻² = n*m²s⁻¹

n*m²s⁻¹ = kgms⁻²

n = kgms⁻² / (m²s⁻¹)

n = kg m¹ ⁻ ² s⁻² ⁻ ⁻¹

 n = kgm⁻¹ s⁻² ⁺ ¹ = kgm⁻¹ s⁻¹

Dimension of n is =  kgm⁻¹ s⁻¹ = ML⁻¹T⁻¹               

Where M = Mass, L = Length , T = Time

It is d, all the other ones say incorrect answer .