Choose the correct word to complete the frantic time traveler's warning.

_____ you touch anything in the year 1920, you might accidentally erase the invention of sliced bread!

The correct answer is If.

"If" introduces the condition that causes the disaster. Using "unless" would mean the disaster happens if you don't touch anything. "Provided that" and "as long as" usually introduce favorable or required conditions, which doesn't fit a warning about erasing sliced bread!