In law, which of these provides testimonial evidence of what he or she knows or claims to know?

No one will witness an incident in exactly the same way as somebody else. If two or more witnesses are telling exactly the same story then something is fishy.

Given the current choices, the current explanation & the question currently worded as it is, both witness & victim could be the answer. Maybe the answer options need to change or perhaps the question could be reworded. Issue Reported.