What uses more electricity - keeping a fridge door open for a while or constantly opening and closing it during this time?

Every time you open your fridge door - or indeed your freezer's door - it allows cold air to fall out onto the floor of the room. If this is allowed to happen for more than a few moments the thermostat inside the fridge or freezer, sensing that too much cold air has gone, will turn on the fridge's motor and compressor pump to cool down the new air. The more the compressor pump and its motor are forced to run unnecessarily, the more electricity would be used, with the consequence that the money to pay for that electricity would simply be wasted.

Knowing those "facts of life" it should be easy to understand why it is better to keep the door closed as much as possible instead of leaving it to hang open for more than the short time it takes to load or unload food from the fridge.