Shanta Sinha is best recognized as an anti-child labour activist, which is option 2. This is because she has spent a substantial part of her life working to prevent child labour and promoting children`s right to education. Therefore, the correctness of this option is very clear and straightforward.

For comprehension of the other options, we can dissect them too. Though important, they do not directly relate to Shanta Sinha`s work. Option 1, referring to a campaigner for urban sanitation, does not align with her work as her main focus is anti-child labour. Option 3 mentions an organizer of rain-water harvesting schemes, which is also not associated with Sinha`s emphasis on child rights. Lastly, option 4, an activist for the welfare of poor rural women, is not precise as she primarily works for the welfare of children and their right to education. That’s why only the second option is accurate.