Finally found work around. Once installer started, I checked mount and mount target mounted ESP on sda. Unmounted that partition & mounted ESP on sdc.
ubuntu-mate@ubuntu-mate:~$ history 1 mount 2 sudo umount /target/boot/efi 3 sudo mount /dev/sdc1 /target/boot/efi 4 mount
Finally found work around.
Once installer started, I checked mount and mount target mounted ESP on sda. Unmounted that partition & mounted ESP on sdc.
ubuntu- mate@ubuntu- mate:~$ history
1 mount
2 sudo umount /target/boot/efi
3 sudo mount /dev/sdc1 /target/boot/efi
4 mount