what is the default content of /etc/apt/sources.list in Ubuntu 18.04 LTS (Bionic Beaver) [duplicate]

As you just want to remove unneeded lines from your sources.list, below is an example of what you could have in the contents of your file:

deb http://in.archive.ubuntu.com/ubuntu/ bionic main restricted universe multiverse
deb http://in.archive.ubuntu.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://in.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu bionic-security main restricted universe multiverse

deb-src http://in.archive.ubuntu.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://security.ubuntu.com/ubuntu bionic-security main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://in.archive.ubuntu.com/ubuntu/ bionic-updates main restricted universe multiverse

This is based on the contents of the sources.list you have provided which have not been commented out. Some things were combined to minimise lines. Additionally, if you don't require source, you can remove all the deb-src lines.

That said, there isn't much of a point to doing this, other than readability. The commented-out sections won't affect anything, and some lines provide information for you. If you definitely won't be needing them, go ahead and use the lines provided above.