Which amendment ensures a defendant cannot be compelled to testify against themselves?

The Fifth Amendment is the correct choice because it explicitly includes a provision known as the right against self-incrimination. This means that an individual cannot be compelled by the government to provide testimony that may incriminate them in a criminal case. This principle helps to protect the fundamental fairness of the legal system, ensuring that defendants have the right to remain silent and not testify against themselves, thereby upholding the presumption of innocence until proven guilty.

The other amendments listed do not provide this specific protection. The Fourth Amendment primarily deals with unreasonable searches and seizures. The Sixth Amendment guarantees the rights of criminal defendants, such as the right to a fair trial, the right to counsel, and the right to confront witnesses. The Eighth Amendment addresses issues related to excessive bail and cruel and unusual punishment. Thus, the protection against self-incrimination is uniquely anchored in the Fifth Amendment.