Step 1: The governing principle is the 4-hour rule of blood banking. Refrigerated blood stays sterile, but once hung at the bedside it warms up and any contaminating bacteria can multiply.
Step 2: Therefore, from the moment the giving set spikes the bag, the transfusion of that unit must be completed in no more than four hours.
Step 3: A practical corollary - blood that has been out of the controlled cold chain for longer than 30 minutes should not be returned to the fridge, and anything not infused within the 4-hour window is discarded for safety.
Step 4: Checking the choices, only the 1 to 4 hour range stays inside the limit, while 3 to 6, 4 to 8 and 8 to 12 hours breach it and invite sepsis.
\[\boxed{\text{1 to 4 hours}}\]