The correct answer is: B. The fire could have been caused by an electrical fault.
An electrical fault is a possible cause of the fire, but it is not certain. There are other possible causes, such as a gas leak or a cigarette butt. More investigation is needed to determine the exact cause of the fire.
Option A is incorrect because it uses the past tense, “had been.” This implies that the cause of the fire is certain, which is not the case.
Option C is incorrect because it uses the simple past tense, “was.” This also implies that the cause of the fire is certain, which is not the case.
Option D is incorrect because it uses the present tense, “could be.” This implies that the cause of the fire is possible, but not certain.