This is a type of exhaust gas after-treatment system used on diesel vehicles that must be visually inspected.

• A. NOx adsorber
• B. Catalytic converter
• C. Diesel Oxidation Catalyst (DOC or OC)
• D. Diesel particulate filter

Answer: (B)

The main idea is recognizing the exhaust after-treatment component that inspectors look for on a diesel vehicle during a visual check. The catalytic converter is that device: it sits in the exhaust stream and uses catalysts to promote oxidation of hydrocarbons and carbon monoxide, helping reduce emissions. On many diesel systems, a DOC (diesel oxidation catalyst) is a type of catalytic converter, so the general item you verify visually is the catalytic converter itself. You’d check that it’s present, properly mounted, intact, and not leaking or damaged. The other options refer to specific or different after-treatment technologies (like NOx reduction devices or particulate filtering) that aren’t the standard item singled out for a basic visual inspection in this context, even though some of them are part of diesel emissions systems.