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.  

it provides an alternative energy pathway for the reaction to proceed.

a catalyst is a substance that speeds up the rate of a chemical reaction but is not consumed during the course of the reaction.

catalysts increase the rate of reaction without being used up. they do this by lowering the activation energy needed. with a catalyst, more collisions result in a reaction, so the rate of reaction increases. different reactions need different catalysts.

a catalyst works by providing an alternative reaction pathway to the reaction product. the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst.