The correct option is D are territorial
Option D: are territorial
According to the lines: "some shark species are territorial and swim around the same area every day. They are only content to be in possession of a part of water but not interested in the movement of other sharks.", some sharks are territorial in nature. Which is why they swim in the same area. In other words, they are restricted to one area.
Hence option D gives the reason required, and is correct.
Option A: share their food with others
"While large sharks allow other sharks to enter their territory, they are not friendly and only share the remains of their food."
As it's mentioned in the above line, the fact that territorial sharks share their food with others, does not give the reason behind why they are restricted to one place.
Option A is not implied by the passage. Hence it is incorrect.
Option B: keep swimming all the time
Only some sharks are restricted to one area, because there are some species that have a territorial trait. They are happy to be in possession of a part of water.
The fact that they have to keep swimming day and night, is another topic altogether. It does not give the reason for them to be restricted in one area. Hence option B is incorrect.
Option C: allow others to enter their territory
This states a feature of territorial sharks. It does not state the reason for them to be swimming in one area only. Hence option C is incorrect.