Final answer:
The entity transferred from Agrobacterium to the plant nucleus that causes the production of the crown gall tumor is DNA.
Step-by-step explanation:
The entity transferred from Agrobacterium to the plant nucleus that causes the production of the crown gall tumor is DNA. Agrobacterium tumefaciens, a bacterium, is able to transfer its DNA to the plant cell's genome, leading to the formation of the tumor. Researchers manipulate the Ti plasmids, which contain genes for tumor production, to remove the tumor-causing genes and insert desired DNA fragments into the plant genome.