Rain is falling at the speed of 4 km/hr from west direction making an angle $30^{\circ}$ with vertical. At what speed should a man walk eastward, such that he has to hold his umbrella vertically to protect himself

Question

$$(a)\;6.518 km/hr\quad (b)\;2 km/hr \quad (c)\;3.464 km/hr \quad (d)4.236 km/hr$$