Final answer:
Organism growth at the cellular level occurs through mitosis and cytokinesis, where cells divide and increase in number.
Step-by-step explanation:
When an organism grows, what happens at the cellular level involves cell division and growth. The correct statement that describes this process is: Cells undergo division through mitosis, increasing the number of cells. During the cell cycle, cells go through interphase where they grow and prepare for division, and then they enter the mitotic phase.
Mitosis consists of several steps - prophase, metaphase, anaphase, and telophase - and is concluded by cytokinesis, where the cytoplasm divides, forming two genetically identical daughter cells. Plant cells and animal cells grow and divide, although they do it in slightly different ways. Cells do not decrease in size or lose chromosomes during growth, nor does cellular metabolism halt; rather, it is geared towards supporting the growth and division processes.