Which of the following is a short-lived free radical bioactive molecule?

Nitric oxide (NO) is recognized as a short-lived free radical bioactive molecule due to its unique chemical properties and biological functions. It is a gas that plays a vital role in various physiological processes, including vasodilation, neurotransmission, and immune response. As a free radical, NO is highly reactive and has a very short half-life, typically on the order of seconds to minutes. This short duration allows it to interact with various targets in the body effectively but limits its stability and long-range signaling capabilities.

The other options do not fit the definition of a short-lived free radical bioactive molecule. Bone is a structural tissue, estrogen is a steroid hormone with a longer half-life and different mechanisms of action, and plasma refers to the liquid component of blood, which contains several long-lived components rather than a short-lived free radical like nitric oxide. Thus, nitric oxide stands out as the correct choice due to its free radical nature and transient existence in biological systems.