Final answer:
Russia threatened to retaliate in response to the US shutting down a Syrian warplane and their warning to stay out of Syrian airspace.
Step-by-step explanation:
The response of Russia to the US shutting down a Syrian warplane and their warning to stay out of Syrian airspace was that Russia threatened to retaliate. In 2013, when the US considered a military strike on Syria, President Obama ultimately allowed Russian President Vladimir Putin to negotiate Syria's surrender of its chemical weapons. This shows that Russia took a strong stance on the issue and did not ignore the situation.