The correct answer is (c) Audio mixers. Summing amplifiers are extensively used in audio mixing applications where multiple input signals from different audio sources need to be combined into a single output. A summing amplifier performs the function of combining multiple input signals with different gains (scaling factor) and producing a weighted sum of those inputs. In audio mixing consoles, summing amplifiers are used to add multiple audio channels together, allowing sound engineers to blend different instruments and vocals into a single mix. Scaling amplifiers adjust the amplitude of signals, and averaging amplifiers compute the average of multiple input signals. All these applications are commonly found in audio mixer systems, making (c) the most practical and widely used application among the given options.