The correct answer is option 2 - Divergent lens.

A double convex air bubble in water is a convex lens. A convex lens is a lens that is thicker in the middle and thinner at the edges. It causes light rays to diverge or spread out when they pass through it.

When light rays pass through a divergent lens, they are refracted away from the center of the lens. This results in the light rays appearing to come from a virtual focal point located on the same side as the object.

In the case of a double convex air bubble in water, the convex shape of the bubble causes the light rays to diverge when they pass through it. Therefore, the bubble behaves as a divergent lens.

Option 1 (Convergent lens) is incorrect because a convergent lens is a lens that is thicker at the edges and thinner in the middle, causing light rays to converge towards a focal point.

Options 3 (Both the above) and 4 (None of these) can be ruled out because the correct answer is option 2 - Divergent lens.

Therefore, the correct answer is option 2 - Divergent lens.