How do I unlock the radio code for my 2004 Chevy Optra?

Contact your local dealer.
Have the Vehicle Identification NUmber (VIN) and proof of ownership available. Many will do this for little or nothing, with or without an appointment.
Be polite. I've more experience with dealer's in more states than most have visited. You'll tame more bears with honey than you will with vinegar. Good luck!