Hey there! I'm happy to help!
Ventral has to do with the front side of the body. The ventriloquism (controlling a puppet and talking without your lips without moving to give the illusion that the puppet is talking), it has to do with creating an illusion from the front side of your body. It comes from a Latin word that means "belly", so something like ventriloquism is "belly speaking". The ventricles of the heart are chambers that are closer to the belly. So, this is not referring to the back region of body and is incorrect.
Gluteal has to do with the gluteus maximus muscles. This is a specific section of the back region of the body and not the actual back region as a whole, so this is incorrect.
Deep just means that something is further inside your body (further from the skin). For example most of your bones are deep to your muscles. This is the incorrect answer.
You may have heard of a dorsal fin on a fish or shark. These fins are normally on the top of a fish. This is the same thing as being on their back because their bellies (ventral, front) are facing down. This term does apply to the back region of the body, so this is correct.
The best anatomical term to describe the back region of the body would be dorsal.
Have a wonderful day and keep on learning! :D