Which of these operations does NOT use the libuv thread pool?

"Reading data from a TCP socket" is correct. Network sockets use the operating system's own non-blocking I/O, so they never use a pool thread:
• epoll on Linux
• kqueue on macOS
• IOCP on Windows

The other options all run on the libuv thread pool. This includes "fs.readFile", "crypto.pbkdf2" and "zlib.gzip". The pool also runs other async crypto functions like scrypt, and dns.lookup. They use the pool for two reasons. Some have no non-blocking OS interface that works on every platform. Others are CPU-heavy.