Final answer:
The most suitable word to replace 'cat' in the sentence is 'D. mouser', as it specifically describes a cat that hunts rodents, which corresponds to the cat's role in the sentence.
Step-by-step explanation:
The best word to replace 'cat' in the sentence “The cat who lives in the barn helps to control the rodent population” is D. mouser.
The term 'mouser' directly refers to a cat that is known for catching mice, which directly relates to the context given in the sentence about controlling the rodent population. While 'tabby' refers to a specific coat pattern, 'kitty' is a diminutive term for a cat, and 'feline' is a general term for any member of the cat family, 'mouser' specifically highlights the cat's role in pest control. Therefore, 'mouser' is the best fit for this sentence as it most accurately describes the cat's job in the barn.