In the given sentence, 'found' indicates that the tense of the sentence is past tense. Thus, the answer must also be in the past tense. Also, 'since' indicates that the action is going on from a point in time in the past till the present moment. Thus, the answer must indicate that the action is continuous.
Option A is incorrect. 'leaked' is in the past tense but, it does not indicate a continuous action. Thus, it is incorrect.
Option B is incorrect. 'is leaking' is in the present continuous tense. Thus, using it will make the tense of the sentence inconsistent.
Option C is incorrect. 'was leaking' is in the past tense and the action mentioned is continuous but, it indicates that the action was ongoing in the past and is now complete. In the given sentence, 'since' indicates that the action is going on till the present moment. As 'was leaking' cannot indicate that, it is incorrect.
Option D is correct. 'had been leaking' indicates that the tank started leaking at a time in the past and is doing so till a point in the present. Thus, it indicates an action that is going on till the present moment and so, it is correct.