Question

Jack is seeing in onion cell under a microscope. He observes formation of a cell plate he is observing which phase of the cell cycle

Answer 1

The answer should be cytokinesis

Answer 2

Jack is observing the CYTOKINESIS STAGE of the cell cycle.
Th cytokinesis stage is the phase at which the cytoplasmic division of a cell occur resulting in separation of two daughter cells. The formation of a cell plate observed in the onion under the microscope is similar to the separation of daughter cells which occur at the end of mitosis.