Explanation:

- The concept of playback singing revolutionized Bollywood music. This allowed singers to record separately from actors, who would then lip-sync on screen.

- R.C. Boral and Pankaj Mullick were pioneers in introducing playback singing.

- 1935 is widely recognized as the year they first recorded a playback song for the Hindi movie "Dhoop Chhaon" (or its Bengali version "Bhagyachakra").

- This innovation marked a shift in the film industry, separating voice from screen performance.

- The other options (1939, 1941, 1943) came later and are incorrect for the first instance of playback singing.

- The timestamps show the continued evolution and impact of playback technology.

Correct Answer: Option 4: 1935