Final answer:
Organogenesis is the process where multicellular organisms develop organs from a single-cell zygote, involving stages like gastrulation and cell specialization, leading to the array of specialized cells in an adult organism. The correct option for the phenomenon of increasing complexity in organism development is Cell specialization (Option 3).
Step-by-step explanation:
The phenomenon in which the development of organs in a multicellular organism involves a sequence of events that increase the complexity of the organism is called organogenesis. This process is characterized by the differentiation of cells into specific tissue types that organize to form organs. During development, a single-cell zygote undergoes various stages such as gastrulation where it forms three germ layers that will give rise to different tissues and organs through a well-regulated process of cellular signaling and gene expression, leading to cell specialization and organization into complex structural and functional units.
Stem cells play a crucial role as they have the potential to divide indefinitely and differentiate into specialized cells under certain conditions. The ability to produce different types of cells and organize them in a three-dimensional pattern is essential in complex multicellular organisms such as humans, allowing for the development from a fertilized egg into a being with an array of specialized cells like nerve cells, muscle cells, and epithelial cells. Please note that the correct option for the phenomenon discussed is Cell specialization (Option 3).