Final answer:
The safest option to avoid turbulence caused by the jet stream with dropping temperatures is to fly above the jet stream, as it tends to minimize encounters with turbulence.
Step-by-step explanation:
The best way to avoid turbulence when flying in conditions where temperatures are dropping and the jet stream is present is by flying above the jet stream. The jet stream can cause significant turbulence due to the high-speed winds and various atmospheric conditions it encompasses. Flying above the jet stream reduces the likelihood of encountering such turbulence. Alternatives like flying below the jet stream or changing altitude frequently might still expose the aircraft to turbulence, and while flying through the jet stream as quickly as possible might minimize time spent in turbulent conditions, it does not necessarily avoid it.