Final answer:
If immersion oil was replaced with water, the resolution would improve compared to using air but would not be as high as when using oil. This is due to water having a refractive index lower than oil but higher than air.
Step-by-step explanation:
If immersion oil was replaced with water, the refractive index of water would be greater than air but less than oil, which means that resolution would improve less than with oil. The refractive index (n) is a measure of how much the speed of light is reduced inside a medium. The immersion oil has a refractive index very similar to that of glass (n = 1.51), which is used in microscope slides and lenses. The refractive index of water is about n = 1.33, which is higher than air but lower than oil.
Replacing oil with water would still decrease the amount of light scattering compared to air, but because the refractive index of water is less than that of oil, the resolution obtained would not be as high. When light passes from the specimen through the glass and into the immersion medium, any difference in refractive indices can cause light to refract, which affects resolution.
Therefore, the correct answer to the question is that if immersion oil was replaced with water, the refractive index of water would be greater than air but less than oil, improving resolution less than oil.