Final answer:
Lighting flashes and large hail can indicate the formation of tornadoes. Softball-sized hail is the likely indication of a highly powerful storm.
Step-by-step explanation:
Lighting flashes at night can be used to define cloud formations that may be tornadic. However, large hail can also indicate that you are near the area where a tornado is most likely to form. The size of the hail can help determine the severity of the storm and the likelihood of a tornado. In this case, the correct answer is D) Softball-sized. Softball-sized hail is a strong indication of a highly powerful and dangerous storm.