Which among the following has its refractive index closest to that of crown glass?

Canada balsam

The correct answer is option 4. Canada balsam.

Among the options provided, the material that has its refractive index closest to that of crown glass is Canada balsam.

Canada balsam is a type of resin with a refractive index relatively close to that of crown glass. This similarity in refractive index is advantageous in optical applications where materials with matching refractive indices are desired for minimizing reflection and maximizing light transmission.