You made a typo in a low-level Linux configuration file that prevents your Compute Engine instance from booting to a normal run level. You just created the Compute Engine instance today and have done no other maintenance on it, other than tweaking files. How should you correct this error?
If the Compute Engine instance cannot boot to a normal run level due to a configuration error, logging in through SSH will not be possible since the SSH daemon may not be running. The serial port provides a way to access the instance at a lower level, which is necessary to fix configuration issues preventing normal booting. Therefore, configuring and logging in to the Compute Engine instance through the serial port to change the file is the correct approach.
If booting issue with Compute engine instance, then serial port access is one of the solution. SSH, RDP and SCP are not possible.
https://cloud.google.com/compute/docs/troubleshooting/troubleshooting-using-serial-console Answer C
C is the answer. https://cloud.google.com/compute/docs/troubleshooting/troubleshooting-using-serial-console
The correct answer is B: Configure and log in to the Compute Engine instance through SSH, and change the file. This is the recommended method to make changes to a Linux configuration file on a Compute Engine instance. SSH allows secure remote access to the instance's command line interface, and it is designed to enable you to make changes to the instance's configuration files. Option A, downloading and uploading the modified version of the file, is not the recommended method as it requires more steps and can introduce errors. Option C, using the serial port, may be used in some cases, but it is not the recommended method as it requires more steps and can be more complex. Option D, using a remote desktop client, is not applicable as Linux instances on Compute Engine do not come with a graphical user interface (GUI) by default.
Computer access through serial ports has been the method used by server administrators for a long time, so it is normal for professionals to be able to do this. If we consider that the computer is not running, there is no correct answer other than C.
According to the explanation "prevents your Compute Engine instance from booting to a normal run level". So I think sshd deamon has not launched yet and you can't use ssh. I can't think of a correct answer to anything other than C.
The only chance is to connect to the console via the serial connection