The speaker is advising to use a method other than "persuasion" to get witnesses to court. The word that is most nearly opposite in meaning to "persuasion" is "coercion."
"Persuasion" involves convincing or influencing someone through reasoning or argument, often with the aim of gaining their voluntary cooperation. On the other hand, "coercion" implies the use of force or threats to make someone do something against their will.
So, the sentence suggests that it is better to use persuasion (convincing or influencing) rather than coercion (force or threats) to get witnesses to court. Therefore, option B, "coercion," is the correct choice.