In artificial hybridisation, bagging is done to:

The correct answer is Option (4) → Prevent contamination of stigma with unwanted pollens

Option (1) Remove Anthers — Incorrect. Removing the anthers from the flower bud is emasculation, the step done before bagging.

Option (2) Prevent unnecessary fruit formation — Incorrect. Bagging is done so that the desired fruit and seed do form, from the chosen pollen only.

Option (3) Ensure cross-pollination — Incorrect. The bag itself does not bring pollen. The breeder dusts the chosen pollen on the stigma by hand and then bags the flower again.

Option (4) Prevent contamination of stigma with unwanted pollens — Correct. In artificial hybridisation the emasculated flower is covered with a bag of butter paper. This bagging keeps unwanted pollen away from the stigma, so that only the chosen pollen pollinates it.

NCERT Extract:
Pollination – “… A breeder is interested in crossing different species and often genera to combine desirable characters to produce commercially ‘superior’ varieties. Artificial hybridisation is one of the major approaches of crop improvement programme. In such crossing experiments it is important to make sure that only the desired pollen grains are used for pollination and the stigma is protected from contamination (from unwanted pollen). This is achieved by emasculation and bagging techniques.”