We have changed the "Debug options" checkbox to a dropdown for controlling the overall optimization level. This allows you to not just switch between debugging or release optimization in the compiler, but specify exactly what sort of optimizations you would like to enable.
Contrary to other CMake build systems, the Zephyr build system uses Kconfig options to control the optimization level, which would often confuse new users. We hope this change makes the change a little smoother for new users.