Which of these powers does the constitution denied the federal government

The answer is: C. Passing laws that apply to actions taken before the law was passed

This is because this power of passing laws that apply to certain actions is a power given exclusively to the Legislative branch of government not in this case the Federal (executive branch).

although the legal justifications for the trials and their procedural innovations were controversial at the time, the nuremberg trials are now regarded as a milestone toward the establishment of a permanent international court, and an important precedent for dealing with later instances of genocide and other crimes