Can I use my D drive as my C drive?

Two possible answers to this question.  Without access to the PC I can not tell you which is correct.

You may have one large hard drive that is divided into two section.  One is the C: where Windows has to reside for the computer to boot up.  You have no choice but to have Windows on the C:.  The other is a 2 TB partition that intended to hold your data files, etc.  Or you have two physical hard drives inside of your computer. One os the C drive and the other the D drive.  (This is only likely if you have a desktop.  Not enough room inside a laptop for two physical drives.

If you have two partitions on one drive, then when you are installing Windows you should be offered an option about where you want to install Windows and how large you want to make the C drive.  You should be able to add that 2 TB of space to the C drive and make it 2 TBs larger.  But you would not longer have a D drive.  (This is not possible if you have two physical hard drives.)

But what most people do is to sue the D Drive to store their data.  They create folders, and save all their music, videos, documents, pictures, etc on the D drive.  This frees up space on the C drive.  And it also makes it easier to back up the computer.  Everything you want to back up is on D drive so you only need to back it up.  And if you get a new computer, all your data is on the D drive and you just copy that to new drive.

Also many programs when you are installing them ask you where you want to install them.  You can create a "Program Files" directory and the D drive and install programs and games there instead of your C drive.  You just need to change the location during the install of the program.  (If you have two physical hard drives, this can make programs run faster because your computer can access the C drive to run Windows files at the same time it is accessing the D drive to run your programs or games.

As you do not sound like you are very tech-savy, probably the better option for you is to store your data files on the D drive, and install programs that will let you on that drive rather than trying to expand the C drive.

basically, you can't.  the operating system and many of your programs use the C drive by default.  some programs can have their "save" locations reset to the D drive ... you'll have to look and see which allow this -- the operating system will not allow this and all the downloads from updating the o/s will end up in the C: drive

Not much, C drive is mostly used by the operating system, old operating system had issue with memory management, no- a-days issue are like this, you give more space to operating system, it will take over the whole space. So its recommended to give additional space to operating system, but not too much.

In your case, if your hard ware supports you can combine or merge, everything as one logical drive, but take care you don't delete anything that is required by OS.