Which of the following factors contributes most significantly to an optical receiver's sensitivity?

The sensitivity of an optical receiver is primarily determined by its ability to detect weak signals in the presence of noise. Noise can originate from various sources and can adversely affect the receiver's performance by obscuring the signal. A higher level of noise means that the receiver requires a stronger signal to distinguish it from the background noise, reducing its sensitivity to weaker signals.

Factors such as distance, physical size, and temperature can have some influence on signal quality and receiver performance, but they do not directly contribute to sensitivity in the way that noise does. For instance, while distance can affect signal strength due to attenuation over long fiber runs, it doesn't inherently alter the receiver's sensitivity itself. Similarly, physical size may relate to design aspects but does not impact the fundamental ability of a receiver to detect signals. Temperature can affect electronic components within the receiver but does not define the sensitivity level as clearly as noise does. Thus, noise is the key factor that primarily affects how sensitively an optical receiver can operate.