Final answer:
The CT number representing water is 0, as per the Hounsfield scale used in CT scans to define the radiodensity of various substances with distilled water set at zero.
Step-by-step explanation:
The CT number representing water is a) 0. In the context of a CT (Computed Tomography) scan, the Hounsfield scale is used, on which the radiodensity of distilled water at standard pressure and temperature is defined as zero. This scale has been designed in such a way that each unit represents a difference of 0.1% of the x-ray attenuation coefficient of water. This provides a quantitative measure that enables the identification of various tissues in a non-invasive manner. Air is represented by -1000 on this scale, and very dense materials such as bone can have CT numbers above +1000.