The best cell for the study of the structure of the nucleus is the cell in the divisional phase.

Reasons:
There are several reasons for this:

1. Chromosomes are Visible: During the divisional phase, the chromosomes are condensed and visible under a microscope. This makes it easier to study the structure of the nucleus and the arrangement of genetic material within it.

2. Mitotic Spindle Formation: The mitotic spindle is formed during the divisional phase, which plays a crucial role in cell division. By studying the formation and organization of the mitotic spindle, researchers can gain insight into the mechanics of cell division and the role of the nucleus in this process.

3. Nucleus is Active: During the divisional phase, the nucleus is actively involved in the process of cell division. This means that there is a lot of activity taking place within the nucleus, which can be studied to gain a better understanding of its structure and function.

4. Variation in Structure: There is a lot of variation in the structure of the nucleus during the divisional phase. This variation can be studied to gain insight into the mechanisms that govern the organization of genetic material within the nucleus.

Conclusion:
Overall, the cell in the divisional phase is the best cell for the study of the structure of the nucleus. It provides researchers with a wealth of information about the organization and function of the nucleus, and can be used to gain insight into a wide range of biological processes.

Cell in interphase