Option B: “They’re” is the shortened form of “They are”. The sentence becomes meaningful with this option. “They are the only ones left on the island.” Hence, this is the correct option.
Option A: “their” is used to indicate possession or ownership, for example, “They went in their car.”. This is meaningless in the given case. Hence, this option is incorrect.
Option C: “there” represents a position, for example, “He was standing there.”. This is meaningless in the given case. Hence, this option is incorrect.
Option D: Since only option A is correct, thus, this option is incorrect.