Image manifest

You can see which packages are pre-installed as mentioned at: How to get a list of preinstalled packages?

E.g., on Ubuntu 18.04, the packages are at: http://releases.ubuntu.com/18.04/ubuntu-18.04.1-desktop-amd64.manifest

Then, since I know that my kernel is located at: /boot/vmlinuz-4.4.0-141-generic, to find the package name I did:

which gave:

So I just search for linux-image- in the .manifest and it gives:

so I conclude that Ubuntu 18.04 comes with Linux kernel 4.15.

packages.ubuntu.com

If you search for the package name on Google: linux-image-4.15.0-29-generic, it also leads us to the packages.ubuntu.com page: https://packages.ubuntu.com/bionic/linux-image-4.15.0-29-generic

Then, on the breadcrumb navigation in that page, there is a link to the "kernels" section: https://packages.ubuntu.com/bionic/kernel/

And by searching for linux-image- in that page, we find several kernels that can be installed in the system.