The correct answer is: D. Wavelet transform
A wavelet transform is a mathematical tool that decomposes a signal into a set of wavelets. Wavelets are functions that have a localized support, meaning that they are zero outside of a certain interval. This makes them well-suited for representing signals that have sharp changes, such as speech signals.
The wavelet transform is one of the most frequently used transforms in speech coding. It is used to represent the speech signal in a compact form, which can then be transmitted or stored. The wavelet transform can also be used to remove noise from speech signals, and to improve the quality of speech signals that have been compressed.
The other options are incorrect because they are not as frequently used as the wavelet transform in speech coding. The Fourier transform is a mathematical tool that decomposes a signal into a set of sine waves. The discrete cosine transform is a mathematical tool that decomposes a signal into a set of cosine waves. The shearlet transform is a mathematical tool that decomposes a signal into a set of shearlets. Shearlets are functions that have a directional support, meaning that they are zero outside of a certain direction. This makes them well-suited for representing signals that have directional changes, such as images.
The Fourier transform, discrete cosine transform, and shearlet transform are all useful tools for signal processing, but they are not as frequently used as the wavelet transform in speech coding.