What is the reason of 'stone-cancer'?

Certain rock-forming minerals are chemically vulnerable to acids: calcium carbonate (CaCO3), the mineral that makes up marble and limestone, reacts with strong or moderately strong acids and converts into different calcium salts, some of which are markedly more water-soluble, so they wash away or flake off and erode the stone's surface over time.

Industrial and vehicular emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) are oxidised in the atmosphere and combine with moisture to form dilute sulfuric acid (H2SO4) and nitric acid (HNO3) respectively - this acidified rainfall is acid rain. When acid rain falls on marble or limestone monuments, the sulfuric-acid component reacts with the surface calcium carbonate: CaCO3 + H2SO4 + H2O -> CaSO4.2H2O + CO2, converting the stone surface into gypsum (calcium sulfate dihydrate), a mineral far more water-soluble than calcite, which flakes off and pits the surface; the nitric-acid component separately converts calcium carbonate into highly soluble calcium nitrate, which is leached away by further rain. Together these reactions progressively eat into the stone the way a malignant growth spreads through tissue - which is why the process is popularly named 'stone-cancer', and why it is the reason behind the disfiguring, pitted corrosion seen on marble monuments such as the Taj Mahal. This makes acid rain the keyed answer.

Ruling out the other options by what each actually is:

• Global warming is a rise in Earth's average atmospheric temperature driven by an enhanced greenhouse effect, working through radiative heat trapping over decades - a climatic shift, not a stone-eroding chemical process.

• Radioactivity is the spontaneous emission of particles or energy from unstable atomic nuclei, damaging materials and living tissue through ionising radiation - a nuclear-physics effect, unconnected to any stone-eroding chemical process.

• Bacterial activity involves microorganisms such as lichen or algae slowly colonising a stone surface - a gradual biological effect, far too limited to explain large-scale monument corrosion.