The most accurate statement regarding why scientists can never prove a theory absolutely is:
Because new evidence or observations could potentially contradict or modify the theory.
Scientific theories are based on the best available evidence at the time, and they are always open to revision in light of new data. This is a fundamental aspect of the scientific method; theories must be testable and falsifiable, meaning that if new evidence emerges that contradicts a theory, that theory may need to be adjusted or replaced. This ongoing process is what allows science to grow and improve our understanding of the natural world.