Consider the statements A, B, C on dolphin and choose the correct answer: A.…

In an "evaluate statements A, B, C" question, each statement must be checked independently against verified facts, and the matching option is the one whose true/false pattern exactly matches the checked pattern. A species is called a bio-indicator (or indicator species) of an ecosystem when its presence, absence, or population health reliably reflects the overall ecological condition of that ecosystem, because the species is especially sensitive to pollution, habitat loss, or other stressors.

Checking each statement about the Gangetic river dolphin:

• Statement A (freshwater habitat): the Gangetic river dolphin (Platanista gangetica), India's National Aquatic Animal, is one of the few obligate freshwater dolphin species in the world and inhabits the river systems of the Ganga and the Brahmaputra (and their tributaries).

• Statement B (sanctuary in Bihar): the Vikramshila Gangetic Dolphin Sanctuary, on the Ganga near Bhagalpur, is located in Bihar and is India's only wildlife sanctuary dedicated to the species.

• Statement C (indicator of river health): because the Gangetic river dolphin is highly sensitive to water pollution, reduced river flow, and habitat degradation, its presence and population trend are used as a bio-indicator of the ecological health of the river, in the same way the tiger is used as an indicator of forest health.

All three statements check out as factually correct, so the option stating that A, B, and C are all correct is the right choice.

Cross-check against the other options:

• "A and C are correct but B is incorrect" fails, because the Bihar sanctuary statement (B) is also factually correct, not false.

• "B and C are correct but A is incorrect" fails, because the freshwater-habitat statement (A) is also factually correct, not false.

• "A and B are correct but C is incorrect" fails, because the indicator-species statement (C) is also factually correct, not false.