Explanation:

- Sulphur: Present in petroleum, sulphur compounds can lead to corrosion by forming acidic by-products during combustion. These acids damage engine parts and refining equipment.

- Correct Answer: Sulphur is indeed the compound that causes corrosion.

- Potassium: Generally, potassium isn't present in significant amounts in petroleum or known for causing corrosion in engines or refineries.

- Sodium: While sodium isn't as prevalent for corrosion concerns in fuels, its salts, if present, can lead to turbine and engine fouling, but not typically corrosion.

- Calcium: This is relatively harmless in terms of corrosion when found in petroleum. Its compounds are used more often as detergents or oil additives.