The answer is Grunt.

The relationship between the first two words is that the first word is an animal that makes a certain noise, and the second word is the noise that the animal makes. The same relationship exists between the third and fourth words.

• Pig is an animal that makes a noise called oink oink.
• Rabbit is an animal that makes a noise called grunt.

• Sty is a place where pigs are kept, not a noise that a pig makes.
• Burrow is a hole that a rabbit lives in, not a noise that a rabbit makes.
• Bunny is a young rabbit, not a noise that a rabbit makes.