Which field indicates that a frame can be dropped if the network is congested?

The field that indicates a frame can be dropped if the network is congested is known as the DEI, which stands for Drop Eligible Indicator. This field is used in Ethernet networking to signal that a particular frame may be discarded during periods of congestion, effectively allowing for Quality of Service (QoS) management. When a network becomes congested, frames marked with the DEI can be dropped without affecting higher-priority traffic, ensuring that more critical data flows without interruption.

In contrast, the other fields have different functions. The PCP (Priority Code Point) is used to prioritize traffic by marking frames with a specific priority level, while the VID (VLAN Identifier) specifies the VLAN to which a frame belongs, and TPID (Tag Protocol Identifier) indicates the presence of a VLAN tag but does not engage in congestion management. Therefore, the DEI is specifically designed for indicating frames that can be dropped if needed, making it the correct answer to the question.