Final answer:
The most likely sources of evidence leading to Fritz Sackel's conviction at the Nuremberg Trials were the United States and Great Britain, as they were prominent members of the International Military Tribunal.
Step-by-step explanation:
The representatives of the prosecution who MOST likely provided the evidence that led to the conviction of Fritz Sauckel at the Nuremberg Trials for Count 3, War Crimes, and Count 4, Crimes against Humanity, are the United States and Great Britain. This is because the International Military Tribunal (IMT) was composed of judges and prosecutors from the United States, Britain, France, and the Soviet Union. It can be inferred that evidentiary contributions came from multiple Allied nations; however, given the strong investigative roles of both the U.S. and British prosecution teams, these two are the most likely sources of the evidence leading to Sackel's conviction.