Final answer:
The correct reference for vertical and horizontal form design that should be kept on site is ACI 318, the American Concrete Institute's Building Code for Structural Concrete.
Step-by-step explanation:
The reference that is typically incorporated by reference for the design of vertical and horizontal forms and required to be kept on site at all times is Option 1: ACI 318. The ACI 318, also known as the American Concrete Institute's Building Code Requirements for Structural Concrete, provides standards for the design and construction of concrete structures. While other options, like ASCE 7, ASTM D638, and NFPA 70, provide standards for other aspects of engineering and construction, ACI 318 specifically covers the requirements for concrete formwork design and construction.