The correct answer is D. 6 meters of water column.
The pressure at a depth of 50 m below the oil surface is equal to the atmospheric pressure plus the pressure due to the weight of the oil column above it. The atmospheric pressure is 0.2 kg/cm2, and the specific gravity of oil is 0.8. The weight of a column of oil 1 cm2 in cross-section and 1 m high is 0.8 kg. Therefore, the pressure due to the weight of the oil column above a depth of 50 m is 0.8 x 50 = 40 kg/cm2. The total pressure at a depth of 50 m is therefore 0.2 + 40 = 40.2 kg/cm2. This is equivalent to a pressure of 40.2 x 100 = 4020 cm of water column. 1 meter of water column is equal to 100 cm of water column, so the pressure at a depth of 50 m below the oil surface is equal to 4020/100 = 40.2 meters of water column.
Option A is incorrect because the pressure at a depth of 50 m below the oil surface is not equal to the atmospheric pressure. The atmospheric pressure is 0.2 kg/cm2, but the pressure at a depth of 50 m is 40.2 kg/cm2.
Option B is incorrect because the pressure at a depth of 50 m below the oil surface is not equal to 3 meters of water column. The pressure at a depth of 50 m is 40.2 meters of water column, which is more than 3 meters.
Option C is incorrect because the pressure at a depth of 50 m below the oil surface is not equal to 5 meters of water column. The pressure at a depth of 50 m is 40.2 meters of water column, which is more than 5 meters.