The correct answer is: B. in parallel.
The two cages of a double squirrel-cage induction motor are connected in parallel. This is because the two cages have different resistances, and the parallel connection allows the current to flow through the cage with the lower resistance. This results in a more efficient motor.
Option A is incorrect because the two cages are not connected in series. If they were, the current would have to flow through both cages, which would result in a higher resistance and a less efficient motor.
Option C is incorrect because the two cages are not connected in series-parallel. If they were, the current would flow through one cage, then through the stator, then through the other cage. This would result in a higher resistance and a less efficient motor.
Option D is incorrect because the two cages are not connected in parallel with the stator. If they were, the current would flow through the stator, then through both cages. This would result in a higher resistance and a less efficient motor.