Euro emission norms are emission standards and these represent a package setting limits for emission from a vehicle. Which of the following gases is covered under this?

The correct answer is D (All the above). Euro norms are a package of limits for tailpipe pollutants, and the package covers carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). Particulate matter (PM) was added for diesel vehicles from Euro 1 onwards and tightened in later stages. Each stage, from Euro 1 in 1992 to Euro 6, tightened these limits in steps.

Since each of A, B and C is covered, a single option cannot be the full answer, so D is correct. The Indian Bharat Stage norms follow the same structure. BS-IV was the national standard in 2013 and BS-VI came in from April 2020, which is a change since the exam year.

Core concept: emission standards regulate pollutants, not greenhouse gases in general. This links to the Mains theme of urban air pollution, vehicular emissions, and the role of fuel quality and emission technology such as catalytic converters.

Trap: aspirants often pick only NOx or only CO, or confuse the norms with CO2 regulation. CO2 is not part of the Euro emission limits, which are set separately through fuel-economy and CO2 targets.