D I was driving too fast.

Hi Tracy 🤗 Sentence “B” is correct (“I think I might have been driving too fast”). Sentence “C” is in the “past tense”, and sentence “B” uses the “past participle”. So why “B” and not “C”? As a native speaker, when I read this dialogue, sentence “B” sounds much more natural. We should choose the “past participle” option when we are talking about a past event that can be linked to another time (usually the present). In your example, the two people are probably talking about all aspects of this car accident (including “present” consequences from the accident such as current injuries, car damage, dealing with car insurance, etc.). Even though the “fast driving” is finished, the “fast driving” is now linked to present circumstances. I hope this helped 🤗🌈 Nicky Ann

B is perfect. C would work for me if you change it to "I think I was driving too fast," because driving was a continuous action you were doing when the accident occurred.