The correct answer is A. Low sulphur content.

Nigerian crude oil is classified as "light" crude oil due to its relatively low density and low sulphur content. Light crude oils typically have:

- API gravity > 31.1° (less dense)

- Low sulphur content (< 1%)

- High API gravity indicates a lower density and a higher proportion of lighter hydrocarbons

Crude oil is called "light crude" when it has a low density, meaning it is less viscous and easier to refine, typically indicated by a high American Petroleum Institute (API) gravity value, which signifies that the oil is lighter than water and will float on it; essentially, it contains a higher proportion of lighter hydrocarbon fractions making it more valuable for producing gasoline and other lighter products compared to heavier crude oils. 

Crude oils are said to be sweet because they have low sulphur content.

The low sulphur content makes Nigerian crude oil more desirable for refining into petroleum products, as it requires less processing to remove impurities.