Final answer:
A characteristic of a Layer 2 loop is that broadcast frames are forwarded back to the sending switch, potentially causing network performance issues like a broadcast storm.
Step-by-step explanation:
One characteristic of a LAYER 2 LOOP is option A: Broadcast frames are forwarded back to the sending switch. In networking, a Layer 2 loop occurs when there are multiple paths between switches that cause a loop in the network. Without proper loop prevention protocols, like Spanning Tree Protocol (STP), switches can continuously forward broadcast frames in an attempt to reach every part of the network, unaware that they are perpetuating a loop. This can lead to abundant network traffic which can drastically reduce network performance or completely halt it, a phenomenon known as a broadcast storm.
Option B, the Time-to-Live (TTL) attribute, is related to Layer 3 operations in networks, where IP packets have a TTL value to prevent them from looping indefinitely. Option C and D are incorrect as well; routers (which operate at Layer 3) don't typically forward frames endlessly between each other without some form of mitigation, and while switches might forward unicast frames, they don't do so 'continually' without a Learning mechanism that eventually resolves the loop issue for unicast traffic.