The question -a single muscle cell can contain dozens of nuclei. which stage of the cell cycle is the cell not completing? how do you know?

I do not understand your question fully because there might be some context missing to it:

Having more nuclei is not something caused by the lack of a process or stage. Some muscle cells usually fuse together, which means they become one. But before they became one, each had their own nuclei. And when they fused, each one kept their nuclei, making one cell with more than one nuclei.

Those cells are called multinucleated cells.

Hope it helped,

BioTeacher101

when a reversible reaction is occurring in both directions at the same rate; the products and reactants do not change in concentration. a chemical reaction where the compound is broken into its constituent parts.