Final answer:
The judge is responsible for deciding what evidence is accepted in court, ensuring trials run correctly, and instructing the jury.
Step-by-step explanation:
The individual who determines what evidence will or will not be admitted in court is A. Judge. In the American court system, judges oversee the trial proceedings and are responsible for ensuring that both parties, the prosecution and defense, act appropriately. They make decisions on the relevancy and admissibility of evidence, maintaining order in the courtroom, and ensuring that the trial adheres to rules of procedure and law. The judge has the authority to exclude testimony that is deemed irrelevant or prejudicial. Furthermore, it is the role of the judge to instruct the jury on the law and how to deliberate on the evidence presented.