It is not necessarily in any boot sector at all and 'D' is meaningless. Suppose I installed Windows OS in my 'D' drive, then where will the Boot-loader get installed? Is it in the Partition Boot sector of D drive or MBR? The Windows boot loader used to be NTLDR on the system partition, but is now WINLOAD on the boot partition. It's WINLOAD that's the Windows NT 6.x operating system boot loader, and that lives in an ordinary file on the Windows boot volume, in the 'System' directory. On Windows NT 6.x, this second program loads Microsoft Boot Manager, which in turn, finally, gets around to loading the Windows operating system boot loader, WINLOAD. 'active') volume, and there's a second bootstrap program in the VBR, of that volume, that is loaded and run by the first bootstrap program. In the old PC/AT and PC98 boot process, there's a bootstrap program in the MBR that looks for a 'startable' (a.k.a. Both by a more lax definition as merely 'program involved in the bootstrap process somehow'.

Neither, really, by a strict interpretation of ' boot loader' as 'operating system loader'. Where does the Boot-loader reside? Is it in the MBR (Master Boot Record) or the Partition Boot sector?

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