Option 1: Debu Choudhuri is well-known in music circles but as an exponent of the sitar, not the flute. He was a legendary sitar maestro who contributed significantly to Hindustani classical music.

Option 2: Madhup Mudgal is a celebrated name in the field of Indian classical music and is well-known as a classical vocalist in the genres of Khayal and Bhajan, not as a flutist.

Option 3: Ronu Mazumdar is indeed well-known as an exponent of the flute. He is an Indian flutist in the Hindustani classical music tradition and is credited with popularizing the instrument both in India and internationally.

Option 4: Shafaat Ahmed, while a respected name in Indian music, is known for his expertise in Tabla, not the flute.

So, the correct answer out of the given options is Option 3: Ronu Mazumdar.