If you are running Windows then yes, if you choose to install Ubuntu on (D:) all you data will be erased. This is because Ubuntu uses a totally different formatting system (ext4) than Windows uses (NTFS). Ubuntu won't run on
NTFS and Windows won't run on
At this point you have two options:
1. You can create a new partition for Ubuntu (shrinking one of your others) and install Ubuntu there. NOTE: if your using a newer computer, you might already have four (4) primary partitions, the maximum a hard drive can support. (There are ways around this, but I don't recommend them for new users)
2. Or you can use the Windows installer (WUBI) to safely install Ubuntu inside of Windows in a kind of virtual partition. If you install Ubuntu this way you can uninstall it like any other Windows program, which is great for beginners.
I recommend using WUBI if this is your first time using Ubuntu and aren't sure you want to commit. Using WUBI eliminates accidental mistakes while partitioning, making it a safer route for beginners.