Can I install Ubuntu 18.04 LTS with 1 GB RAM. My desktop computer has 2.99 GHz 32-bit CPU and 1GB RAM. What Ubuntu version should I install?
Barely, or no. Use Xubuntu.
To install and use Xubuntu, you need at least 512 MB of memory. https://xubuntu.org/requirements/.
It's based on debian and ubuntu: https://www.distrowatch.com/table.php?distribution=xubuntu