The substance produced inside the leaf disks that caused them to float to the surface of the solution is
(1)ozone
(2)water
(3)oxygen
(4)nitrogen

(3) oxygen

Explanation:

The process by which leaf disks float to the surface is as follows:

-Photosynthesis converts the carbon dioxide to oxygen

- Oxygen occupying the gas spaces in the leaf causes the leaf to rise.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.