Which code would require all but emergency traffic to be held?

The code that requires all but emergency traffic to be held is Signal 100. This code is typically used during critical incidents, such as a major emergency response or a serious incident where the information flow needs to be controlled to avoid congestion on communication channels. By holding non-emergency traffic, responders can ensure that vital communications related to the incident can be made without interference, allowing for a more efficient and effective emergency response.

The other options, while they may pertain to different scenarios or types of incidents, do not convey the specific directive to hold all non-emergency communications. For instance, Signal 60 often pertains to a specific type of emergency or operational directive, while the 10-90 code and Priority 1 may represent different levels of urgency or types of calls, but none explicitly require the cessation of non-emergency traffic like Signal 100 does.