My pick is number 2, as gravel is a key ingredient of concrete.
Many people use the terms cement and concrete interchangeably, but that’s wrong. Cement is just one ingredient of concrete; it’s the “glue” that holds the aggregates — that’s a fancy term for “little bitty bits of hard stuff” — together. Those aggregates are typically sand and gravel.
Half of all gravel is used for concrete, and since 2006, humanity has been producing enough concrete to create a cube that’s 1.2 miles by 1.2 miles by 1.2 miles every year. Providing the gravel for even a small fraction of the annual concrete production at with a zero cost of goods sold would be pretty lucrative!
My backup pick would be number 3, because most locked doors and gates, as well as many walls and fences, would no longer be an obstacle.