Option 1 - Aurobindo Ghosh: Aurobindo Ghosh was an Indian nationalist and philosopher, but he was not a pioneer of social reform movements in 19th century India. He played a significant role in the Indian independence movement.

Option 2 - Raja Ram Mohan Roy: Raja Ram Mohan Roy is the correct answer. He was a social reformer and is known as the pioneer of social reform movements in 19th century India. He fought against various social evils such as sati, child marriage, and caste system. He also advocated for women`s rights and education.

Option 3 - Devendra Nath Tagore: Devendra Nath Tagore was a Brahmo Samaj leader and played a crucial role in the Brahmo Samaj reform movement. However, he was not the pioneer of social reform movements in 19th century India.

Option 4 - Keshav Chandra Sen: Keshav Chandra Sen was also a Brahmo Samaj leader and contributed to social reform movements in India. However, he was not the pioneer of social reform movements in 19th century India.

In conclusion, Raja Ram Mohan Roy stands out as the pioneer of social reform movements in 19th century India.