What is the function of a phototransistor?

The function of a phototransistor primarily revolves around its ability to conduct current when exposed to light, which is why the correct answer is that it conducts current when illuminated. A phototransistor is a type of transistor that is sensitive to light; it uses photons to generate carriers that allow current to flow through it. This property makes it particularly useful in applications where light signals must be converted into electrical signals, such as in optical sensors and light detection equipment.

While some of the other choices may reflect functions of different electronic components, they do not describe the specific functionality of a phototransistor. For example, converting sound signals into electrical signals pertains more to microphones or audio sensors rather than phototransistors. Similarly, although phototransistors can act as switches in low-power applications, this is not their primary function; that specific action is more characteristic of typical transistor or relay operations. Lastly, while amplification may be a property of some transistors, phototransistors are not typically used to amplify audio signals in the same manner that dedicated audio amplifiers do. Thus, the defining characteristic and primary function of a phototransistor is its ability to conduct current in response to light exposure.