The correct answer is A) check the transformer ventilation fans for proper operation. The reasoning behind this is that the first action should be to check the primary cause of the rising transformer core temperature, which is likely inadequate cooling. By verifying the proper operation of the transformer ventilation fans, you can quickly identify and address the root issue before it leads to more serious problems. The other options are not the best first actions - notifying the bridge (B) or reducing load (C) may be necessary later, but first you need to determine the cause of the temperature rise. Sending the oiler to look for fires (D) could be an appropriate later step, but should not be the initial action.