The correct answer is option B: 2 and 3 only.

Ganges River Dolphins, also known as South Asian River Dolphins, are a critically endangered species found in the Ganges, Brahmaputra, and Meghna river systems of India, Nepal, and Bangladesh. While poaching has been a significant factor in their decline, other human activities have also contributed to their population decline.

Construction of dams and barrages on rivers (option 1) has led to habitat fragmentation and reduced water flow, which affects the dolphin`s ability to navigate and find food. While crocodiles (option 2) may compete with dolphins for food, there is no evidence to suggest that an increase in crocodile population has directly led to the decline in dolphin populations.

Ganges River Dolphins often get trapped in fishing nets accidentally (option 3), and this is a major threat to their survival. Additionally, the use of synthetic fertilizers and other agricultural chemicals in crop fields in the vicinity of rivers (option 4) has led to increased pollution and contamination of river water, which has negatively impacted the dolphin`s habitat and food sources.