There are many reasons why animals defend their territories. For a good bit of animals, the reason that they defend their territories is to gain and protect their food resources, mating areas, their habitat, and much more. They don't really protect their territories to establish 'dominance' within a social group. They don't do it to eliminate the need for social hierarchies. They don't do it for the reason to increase their space either - although some animals do this sometimes.
Therefore, the correct answer would be the first option you've provided, in my eyes. They do it to maintain access to food, shelter, and mates. If you have any further questions, please be sure to ask me! I'll be more than happy to help!
A) to maintain access to food, shelter, and mates.
