Answer:
B. the witnesses' testimonies
Step-by-step explanation:
A. the witness's testimonies
The underlined is a singular form of a possessive common noun. It should be in a plural form, considering the bolded text above( testimonies).
B. the witnesses' testimonies
This is the correct option. The undelined is properly personalized and it is also pluralized to match the bolded text above.
C. the witness' testimonies
Same as option A.