Answer
It is according to the characteristics of a binomial random variable, page 196.
Work Step by Step
1. We have a sample of 5 bottled-water brands, each one representing a trial.
2. Success: the brand uses tap water. Failure: the brand doesn't use tap water.
3. $p=.25$ and $q=1-p=.75$
4. The result of each brand is independent of the result of the other brands.
5. $x$ is the number of brands that use tap water.
