The given question describes a situation where a substance acts as a convex lens in air and as a concave lens in water.

Option 1 states that the refractive index of the substance is smaller than air. This option is not correct because if the substance is behaving as a convex lens in air, it means that light rays are converging. In this case, the refractive index should be greater than air, not smaller.

Option 2 states that the refractive index of the substance is greater than both air and water. This option is not correct because if the substance acts as a concave lens in water, it means that the light rays are diverging. In this case, the refractive index should be smaller than water, not greater.

Option 4 states that the refractive index is almost equal to water. This option is not correct because if the substance acts as a convex lens in air, the refractive index should be greater than air. It cannot be equal to water.

Option 3 states that the refractive index is greater than air but lesser than water. This option is correct. If the substance behaves as a convex lens in air and a concave lens in water, it means that the refractive index is greater than air but lesser than