In high-performance Data Center networks, what is 'Head-of-Line Blocking' (Switch level)?

1. A When a packet at the front of an input queue is delayed by a busy output port, it forces all packets behind it to wait, even if their intended output ports are completely idle.
2. B When the underlying CPU is unable to calculate the optimal routing path for high-frequency data packets fast enough, leading to a massive backlog of unproccessed segments.
3. C When a physical network cable is suddenly disconnected, causing a momentary 'blocking' state where no data can be transmitted or received across the entire local subnet.
4. D When the BGP protocol fails to converge across multiple disparate autonomous systems, leading to a localized routing failure where all data packets are effectively trapped in a loop.