Quality of Life" (6x09), we meet the exocomps. These little machines are assumed to be disposable non-living machines, until Lt. Cmdr. Data demonstrates that they're alive.
What is the first sign of life he sees? An exocomp was sent into a plasma conduit to complete a repair. It leaves the conduit without finishing the job and won't go back inside. Soon after, the conduit explodes - something the exocomp knew would happen but no one else detected.
In other words, it recoiled from danger. It recognized a threat to its life and pulled back. Data recognizes that as a sign of life - something I hope we learn sooner than the 24th century. When they're put in danger again, Data locks out the transporter to prevent the exocomps from being forced to die. Captain Picard calls it "the most human decision you've ever made." It's also a very pro-life decision and, as Data points out, not an easy one to make.
Later, Data is willing to sacrifice himself to save Picard and Geordi. Here he gives us a real meaning for "choice". He is willing to sacrifice himself - to give of his own free choice - but not to let the exocomps die without the same.