Final answer:
An egg is unicellular because it is composed of a single cell, which is the starting point for all animal life. Including humans, all animals start life as a zygote, which is a unicellular fertilized egg.
Step-by-step explanation:
An egg is considered unicellular because it is made of one cell. This is true for all animals that start life as a fertilized egg, which is a single cell called a zygote. The fertilized egg or zygote undergoes cell division trillions of times to grow into a complex, multicellular organism. For example, the largest cell known is the ostrich egg, which is a single cell that can be seen without a microscope. Despite the size differences between various organisms' eggs, they all share the common characteristic of being a single cell at the beginning of life.