What key step in mitosis does not occur in the last round of cell division to produce the n+n central cell?

The correct answer is cytokinesis.

Explanation:

The process of differentiating a cell into two daughter cells is known as the cell cycle. It is categorized as S (synthetic), M (mitotic), Gap 1 and Gap 2 phases. During the cell cycle, the cell goes through cellular procedures like genetic duplication, growth, and division.  

The central cell (n+n) refers to a diploid cell that amalgamates with a sperm cell to produce a triploid (3n) mass and further gives rise to endosperm. The secondary nucleus or the central cell possesses two polar nuclei in a single cell.  

In angiosperms at the time of the generation of female gametophytes, one nucleus from each pole goes towards the center and combines to produce a secondary nucleus. In the mentioned case, the essential step of mitosis, which would get stopped to produce (n+n) central cell would be cytokinesis.