I installed two OSes, Windows 7 and Linux. Now when I merge my disk partition D and F the grub> prompt is opened when I start my PC. I need to enter four commands to boot into Windows.
I want to uninstall Linux (which is installed in some logical drive), but I have no clue on how to find the drive and remove Linux from there.
I'm building on @mnmnc's comments, here.
What you are seeing isn't really Linux per se, it's GRUB's boot prompt. GRUB is the boot loader commonly used for Linux on x86 and compatible architectures these days, but it's not the only way to bootstrap the Linux kernel. Before GRUB became mainstream and stable, I believe LILO (the Linux Loader) was the most common. For bootstrapping from DOS (yes, this was once reasonably common) one would use LOADLIN. I'm sure there have been others as well, even if we restrict ourselves to the x86/IA-32 architecture.
When you installed Linux, the installation placed GRUB in two distinct locations. The bare-bones boot loader gets written directly to disk, and additional functionality for it (such as that to display a fancy menu, the configuration specifying the menu options available, and the code that lets you choose between them) probably went into /boot/grub on your Linux root partition. Since those files are now no longer available, you have to enter the boot commands manually. Hence the grub> prompt.
The (relatively) simply fix, as has been pointed out in the comments to the original question, is to use your Windows installation media to perform a repair installation. This will overwrite the boot sector part of GRUB, for all intents and purposes "uninstalling" GRUB. The fact that Windows has no understanding of alternate boot loaders and most often simply overwrites them without even prompting is a common failure case when setting up dual boot (the easy way around that is to always install Windows first, followed by any other OSes one wants to run in a multi-boot setup), but in this case you can use it to your advantage to knowingly revert to a single-OS configuration fairly easily.
Once all remnants of your Linux installation have been removed, you can safely remove the partitions that previously held that installation using Disk Management, and then expand your Windows partition(s) into the then free disk space.
TL;DR: Do a Windows repair installation to get rid of the boot prompt.
compmgmt.mscin Windows. You will see some Uninitialised space or some drives that cannot be accessible from Windows. Format them and you will be able to use it, or format and extend existing partitions to use the space on other drive/partition as well.