The size of the Respirable Suspended Particulate Matter [R.S.P.M] in the air is:

The answer is D: RSPM refers to suspended particles small enough to be inhaled deep into the respiratory tract, and the traditional definition in the question's framework is particles less than 5 μm in size. Particles of this size can reach the lower airways and alveoli, which is why they matter for health.

Why the others fail:

• A, B and C describe particles larger than 5 μm. Coarse particles of this size are mostly trapped in the upper respiratory tract and settle out of the air faster, so they are not what the term 'respirable' describes.

Core concept: classification of particulate matter by size (TSP, RSPM/PM10, PM2.5) and its health effects. For Mains, link this to the National Ambient Air Quality Standards and the National Clean Air Programme.

Be careful with current facts. CPCB now uses PM10 (particles below 10 μm) as the regulatory parameter for RSPM, and PM2.5 is monitored separately. The key follows the older 5 μm convention, so check the current definition before using it in an answer.

Trap: aspirants pick the option with the largest-looking cutoff, or confuse RSPM with PM2.5 and choose a much smaller size.