If $p$ is the density and $\eta$ is coefficient of viscosity of fluid which flows with a speed $v$ in the pipe of diameter $d$, the correct formula for Reynolds number $R_e$ is

Reynolds number is given by, $R_e=\frac{\rho vd}{\eta }$, where, $v=$flow velocity, $d=$hydraulic diameter of the pipe, $\rho =$density of the fluid & $\eta =$coefficient of viscosity of the fluid.

Reynolds number is the ratio of inertial forces to viscous forces within a fluid which is subjected to relative internal movement due to different fluid velocities.