To determine which string instrument has no frets and is played using a bow, we need to analyze each option provided:

1. Tambura: This is a traditional long-necked string instrument used in Indian classical music. It usually has frets and is not played with a bow. It is typically used to maintain the droning background during performances.
2. Tanpura: Similar to the tambura, the tanpura is a plucked string instrument used in Indian music to provide a harmonic drone accompaniment. It has no frets but is not played with a bow.
3. Sarangi: The sarangi is a bowed, short-necked string instrument of North Indian music. It does not have frets and is played with a bow, producing a sound that closely resembles the human voice. Hence, this is the instrument we are looking for.
4. Sitar: The sitar is a popular stringed instrument known for its rich sound in Indian classical music. It has frets and is played by plucking the strings with a plectrum, not with a bow.

Based on the analysis above, the correct answer is the Sarangi, as it is the only instrument among the options that has no frets and is played using a bow.