The correct answer is option 4 - James Prinsep.
James Prinsep was an English scholar and antiquary who played a significant role in deciphering the Brahmi and Kharoshthi scripts. These ancient scripts were used in the Indian subcontinent and were vital for understanding and interpreting the ancient inscriptions found in India.
Prinsep`s work in decoding these scripts was groundbreaking in the field of Indology. He was able to determine the phonetic values of the individual signs in the Brahmi script, which allowed for the translation of many inscriptions found on pillars, rocks, and other artifacts. This milestone in decipherment greatly contributed to our understanding of ancient Indian history and culture.
While the other options - Piyadassi, Colin Mackenzie, and Alexander Cunningham - were also notable figures in the field of Indology, they did not decipher the Brahmi and Kharoshthi scripts like James Prinsep did.