Which type of sign is used to inform drivers of highway and street work?

The correct choice is diamond-shaped signs because these signs are specifically used to alert drivers about road conditions or hazards, including highway and street work. The diamond shape is universally recognized for warning signs and indicates that there may be a change in the road's environment or that drivers should exercise caution. This can include unexpected conditions, construction zones, or maintenance work ahead, which requires extra attention to ensure the safety of both drivers and workers.

Rectangular signs generally convey regulatory information or guide drivers, while round signs are often reserved for railroad crossing warnings. Triangle signs are typically used for yield indications. Therefore, the diamond shape's specific association with warning and caution makes it the appropriate sign type for communicating the presence of street or highway work.