Which of the following devices is passive in nature?

The star coupler is considered passive because it does not require an external power source to operate. Its primary function is to split optical signals among multiple output channels without amplifying or modifying the signal. Essentially, it directs light from one input fiber to several output fibers based on the design of its passive optical components.

In contrast, devices like the optical time domain reflectometer perform active functions such as sending and receiving signals to measure characteristics of the fiber, which require power to operate. Similarly, breakout cables, while they might seem passive in function, still rely on the active management of connections within networks. X couplers, depending on their design, might also involve actively managed signals or light paths, making them not strictly passive devices.