Why is carbon often referred to as the "key to life"?

Carbon is often referred to as the "key to life" due to its unique bonding properties. Carbon atoms can form four covalent bonds with other atoms, which allows them to create a wide variety of complex molecules essential for life, including carbohydrates, proteins, lipids, and nucleic acids. This versatility in bonding enables carbon to connect with various elements, including hydrogen, oxygen, nitrogen, and others, leading to the formation of diverse biological molecules necessary for cellular structure and function.

This capacity for forming stable yet diverse molecular structures is fundamental to the complexity of life. The result is a vast array of organic compounds that have different shapes and functions, contributing to the intricate biochemical processes that sustain living organisms. The other options, while potentially relevant in other contexts, do not capture this critical aspect of carbon's role in biology.