Final answer:
The depth of a control joint in a concrete slab should be at least 1/4 of the slab's thickness and can vary based on local building codes.
Step-by-step explanation:
The depth of a control joint in a concrete slab is important to assure the slab cracks in a straight line in the places you want it to crack. The general rule of thumb is that the joint should be at least 1/4 of the thickness of the slab. So, for a slab that is 4 inches thick, the control joint should be at least 1 inch deep. However, the exact depth may vary based on the specifics of a project and local building codes, so it's important to consult these regulations. In summary, answer option d, 'Varies based on local building codes,' is the most accurate, although 1 inch is a common depth for a standard 4-inch slab.