The word "Adivasi" was used for the first time to refer to tribal people by Option 2: Thakkar Bappa. Thakkar Bappa, also known as Thakkar Bapa, was an influential social worker and reformer who dedicated his life to the upliftment and welfare of tribal communities in India.

Thakkar Bappa recognized the need to differentiate and address the distinct cultural and socio-economic characteristics of India`s tribal communities. He believed that the term "Adivasi" (meaning "original inhabitants" or "indigenous people") accurately reflected the historical and cultural identity of these communities.

Thakkar Bappa advocated for the rights of Adivasis and worked towards their empowerment. He founded the "Adivasi Seva Sangh" in 1938, an organization focused on the development of tribal communities. Through his efforts, he aimed to address the issues of social discrimination, economic exploitation, and cultural marginalization faced by Adivasis.

It is important to note that while Thakkar Bappa was the first to use the term "Adivasi" in this context, the concept of recognizing and respecting the indigenous status of tribal communities had been discussed and advocated by several other social reformers and leaders.

Mahatma Gandhi, Option 1, also played a significant role in the upliftment of Adivasis. He believed in the principles of Swaraj (self-governance) and Swadeshi (self-reliance) and worked towards the economic empowerment of marginalized communities, including Adivasis. Although Gandhi used the term "Adivasi" to refer to tribal communities, he did not coin the term himself.

Jyotiba Phule, Option 3, was a social reformer and activist who fought against caste-based discrimination in India. While he advocated for the rights and welfare of marginalized communities, including Adivasis, there is no evidence to suggest that he used the term "Adivasi" specifically to refer to tribal people.

B.R. Ambedkar, Option 4, was a prominent social reformer, jurist, and the architect of the Indian Constitution. While Ambedkar worked extensively for the rights of Dalits (formerly known as "Untouchables"), he also emphasized the importance of addressing the concerns of Adivasis. However, there is no evidence to suggest that he was the first to use the term "Adivasi" in reference to tribal communities.

In conclusion, Thakkar Bappa, also known as Thakkar Bapa, was the individual who first used the term "Adivasi" to refer to tribal people. His efforts and advocacy contributed significantly to the recognition and upliftment of India`s Adivasi communities.