Correct answer: (3) are abundant in human intestines.

Coliforms are used as indicator organisms of sewage pollution because they originate primarily from the intestines of humans and other warm‑blooded animals, so their presence in water strongly suggests fecal (sewage) contamination.

Option-by-option explanation:

• (1) are pathogens.
  Many coliforms (like most strains of Escherichia coli) are not pathogens; they are generally harmless commensals. They are used as indicators precisely because they are common and easy to detect, not because they are always disease-causing. So this is not the main reason.

• (2) ferment lactose.
  Coliforms do ferment lactose with acid and gas formation, and this trait is used in lab tests to detect them. However, lactose fermentation alone is not why they indicate sewage pollution; other lactose-fermenting bacteria can exist in the environment too. So this is only a diagnostic feature, not the core reason.

• (3) are abundant in human intestines.
  This is the key point. Coliforms live in high numbers in the intestines and are shed in feces; thus, if they are found in water, it signals entry of fecal material and hence possible sewage contamination. Therefore this is the correct answer.

• (4) grow within 24 hours.
  Coliforms can be cultured and counted within about 24 hours, which makes them convenient indicators in practice, but fast growth alone is not the defining reason for their use as pollution indicators.