Final answer:
The reasonable domain for the kitten's weight growth from birth to reaching its maximum weight and until death at age 18 is [0, 18] years.
Step-by-step explanation:
A reasonable domain for the weight of a kitten that is born weighing 3 pounds and reaches a maximum weight of 9 pounds before it dies at age 18 would be its age in years since we are looking at the growth over its lifetime. We start measuring from the kitten's birth, so the lower bound of the domain would be 0 years.
The upper bound of the domain is the kitten's lifespan, which is given as 18 years. Therefore, the domain for the kitten's age as it relates to its weight gain over time would be [0, 18] years.
To clarify this in mathematical terms, the domain is all the values that can be input into a particular function, which refers to the independent variable. In this context, the independent variable is time, measured in years of the kitten's life.