The question is asking about the approximate volume of one barrel of oil.

Option 1 suggests that one barrel of oil is approximately equivalent to 131 litres. This is incorrect; a barrel of oil is generally considered to be larger than this.

Option 2 is the correct answer, suggesting that one barrel of oil is approximately equivalent to 159 litres. This is accepted as the standard volume of a barrel of oil in the petroleum industry.

Option 3 proposes that one barrel of oil is approximately equivalent to 257 litres. This is also incorrect as it significantly overstates the volume of a barrel of oil.

Similarly, Option 4 is incorrect. It suggests that one barrel of oil is approximately equivalent to 321 litres, which vastly exceeds the recognized standard volume of a barrel of oil.

In conclusion, one barrel of oil is approximately equivalent to 159 litres, making Option 2 the correct answer.