Which aspect of BGP-LS makes it scalable in large network when multiarea topology information must be gathered?

Transmit flow control is not a defined BGP-LS mechanism and does not describe a scalability feature specific to BGP-LS operation.

Open-loop flow control provides no receiver feedback to the sender, which is the opposite of TCP's closed-loop behavior and would reduce reliability rather than improve scalability.

Hardware flow control uses physical signaling such as RTS/CTS on serial interfaces and has no relevance to BGP-LS, which operates at the application layer.

BGP-LS uses TCP as its transport, inheriting TCP's sliding-window flow control and congestion control mechanisms. This ensures that large volumes of link-state information from multiple IGP areas can be delivered to an SDN controller without overwhelming either endpoint, making BGP-LS well-suited for collecting topology across large, complex networks.