In GRUB’S configuration file you find the line:
root (hd1,4)
What is corresponding device name on a Linux system?

A.
/dev/hdb4

B.
/dev/hda4

C.
/dev/hda5

D.
/dev/hdb5

Explanation:
hd0 /dev/hda Primary Master
hd1 /dev/hdb Primary Slave
hd2 /dev/hdc Secondary Master
hd3 /dev/hdd Secondary Slave

Similarly partition number starts from 0 like array index in programming.