Environmental Issues

Biochemical oxygen demand (BOD) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by micro-organisms.
A higher BOD value means that the water sample is more polluted by biodegradable organic matter.
Distillery effluent (spent wash) is extremely rich in organic matter and has the highest BOD of the four, of the order of 40,000 mg/L.
Sugar mill effluent also carries a heavy load of sugars and other organic matter, so its BOD is high, though far below that of distillery effluent.
Domestic sewage and paper mill effluent have much lower values; taking the figures used in the source, paper mill effluent is the lowest and sewage comes next.
So the ascending order of BOD is PE < S < SE < DE.

Long exposure to fine dust in workplaces can damage the lungs. Fine particles (smaller than 2.5 $\mu m$) reach deep into the alveoli, causing inflammation and fibrosis.
Cement dust contains silica, chromium compounds and other irritants. Workers in cement factories are exposed to heavy dust and have an increased risk of chronic lung diseases and lung cancer.
(Coal mining mainly causes coal workers' pneumoconiosis or black lung, and the link to cancer is weaker.)
So, as per the answer key, the reason of lung cancer among these is the cement factory.

Relative biological effectiveness (RBE) describes how much biological damage a given dose of an agent causes compared with a reference.
The term is used mainly for ionising radiation: different kinds of radiation (alpha, neutrons, X-rays) cause different amounts of damage for the same energy absorbed.
As per the answer key, the cause of such damage is taken as pollution (radioactive and other pollutants).
(The term relative biological effectiveness strictly refers to radiation, so radiation is also a strong candidate.)
So, as per the answer key, the answer is pollution.