Carbon is often grouped by “colour” according to the source from which it is released into the atmosphere.

Black carbon is soot released mainly from burning fossil fuels such as diesel and coal, so (a) matches 4. Grey carbon is the carbon emitted from industrial activity, so (b) matches 3.

Brown carbon is the light-absorbing organic aerosol produced by incomplete combustion of organic matter such as biomass and wood, so (c) matches 2. Red carbon refers to carbon linked to biological particles, such as algae, on snow and ice, so (d) matches 1.

The matching is therefore (a) 4, (b) 3, (c) 2, (d) 1. Every other code starts black carbon at 3 or 1, which ties it to industrial emissions or to snow-and-ice particles instead of fossil fuels.

(a) 4, (b) 3, (c) 2, (d) 1 option is correct.