Final answer:
Both the severity of the Due Process violation and the witness's ability to identify the suspect are considered in deciding whether a person should testify at trial after viewing a lineup that violates Due Process.
Step-by-step explanation:
The decision on whether a person should testify at trial after viewing a lineup that violated Due Process is based on both the severity of the Due Process violation and the witness's ability to identify the suspect. It is a combination of factors a) and b).