What library does ld option -lrt refer to (Bionic libc)?
When you see a -lsomething, the library name is libsomething. Doing a man librt says the following:
NAME librt, libposix4 - POSIX.1b Realtime Extensions librarySYNOPSIS cc [ flag... ] file... -lrt [ library... ]DESCRIPTION Functions in this library provide most of the interfaces specified by the POSIX.1b Realtime Extension. See stan- dards(5). Specifically, this includes the interfaces defined under the Asynchronous I/O, Message Passing, Process Scheduling, Realtime Signals Extension, Semaphores, Shared Memory Objects, Synchronized I/O, and Timers options. The interfaces defined under the Memory Mapped Files, Process Memory Locking, and Range Memory Locking options are pro- vided in libc(3LIB) See the man pages for the individual interfaces in section 3RT for information on required headers. The name libposix4 is maintained for backward compatibility and should be avoided. librt is the preferred name for this library.
-l is the flag used to link a library into your final executable. In this case, the library in question is librt.a (or librt.so or librt.dylib, depending on your system). librt is the realtime extensions library.