Answer:
B. Two witnesses in the case will give their testimony tomorrow.
Step-by-step explanation:
As you may already know, pronouns are words that replace the nouns mentioned above, referring to them at the moment when they do not necessarily need to be reused, within the sentence. This reference that the pronouns make is directly linked to the concept of pronoun-antecedent agreement, since the pronoun must agree in gender, number and degree with the noun he is referring to and which was said before it.
Option C, shown in the question above, is an example of how this should be done, as it shows the pronoun "their" in correct agreement with the antecedent "witnesses".