What specifically causes TCP Head-of-Line (HOL) blocking?

1. A The strict bandwidth limitations of the physical medium, which prevent multiple disparate data streams from being transmitted simultaneously across a single fiber-optic link.
2. B TCP's guarantee of in-order delivery; if a single packet is lost, the kernel must buffer all subsequent data and refuse to release it until the gap is filled by a successful retransmission.
3. C The inherent processing delay within intermediate routers, which must fully reassemble and inspect every packet before determining the most efficient path to the next hop.
4. D The sequential nature of the standard SSL/TLS handshake, which prevents any application-layer data from being transmitted until the cryptographic key exchange is fully completed.