A variable, which is controlled or changed in a scientific experiment to examine the influences on the dependent variable is known as an independent variable. On the other hand, a dependent variable refers to the variable, which is being measured and tested in a scientific experiment.
In the given case, one is observing that how the amount of salt in a mouse food influences the water the mouse drinks. Thus, the amount of salt in the food in the given case is an independent variable, as one can determine the amount of water consumed by the mouse, by manipulating the amount of salt intake given to the mouse.
Hence, the correct answer is option B.