The correct answer is: saw.
The simple past tense is used to talk about events that happened at a specific time in the past. In this case, the event is “I saw an accident today.” The simple past tense is also used to talk about events that happened repeatedly in the past. For example, “I saw that man every day.”
The present perfect tense is used to talk about events that happened at an indefinite time in the past or that have a connection to the present. In this case, the event is “I don’t know the man involved.” The present perfect tense is also used to talk about events that have just happened. For example, “I have just seen an accident.”
The past perfect tense is used to talk about events that happened before another event in the past. In this case, the event is “I saw an accident today.” The past perfect tense is also used to talk about events that happened before a specific time in the past. For example, “I had seen that man before I met him.”
In this case, the simple past tense is the most appropriate tense to use because the event is specific and happened at a specific time in the past.