What device is used to convert energy from one form to another, such as in Ecco sounding equipment?

The correct choice is the transducer, which is used in various applications to convert energy from one form to another, such as in Ecco sounding equipment used for measuring underwater depth and other sonar applications. Transducers can convert acoustic energy (sound waves) into electrical energy and vice versa, making them integral to sonar and echo sounding systems.

When a transducer emits sound waves, it creates pressure fluctuations in the medium (usually water), which can then be reflected off the sea floor back to the transducer. The device captures these reflected sound waves and converts them back into electrical signals, allowing the measurement of depth based on the time it takes for the sound waves to return.

Other options like a transponder are typically used for communication, transmitting signals rather than converting energy. Transformers are designed for changing the voltage of alternating current in electrical systems and do not relate to the context of converting sound waves. Transmission lines refer to the infrastructures used for conveying electrical power or signals over distances and do not perform energy conversion. Hence, the role of a transducer aligns perfectly with the function described in the question.