Nathan has set his cell phone ring tone to be his favorite song. He always keeps his cell phone in his back pocket. Now when he hears the same song on the radio, he puts his hand in his back pocket. The act of reaching into his back pocket every time he hears the song on the radio would be considered a(n) ___________ response.

Conditioned response can be defined as a learned response to the neutral stimuli that are previously responded to. The conditioned response is a part of the Classical conditioning theory that was developed by Ivan Pavlov.

In the given case, Nathan's response to check his phone every time he hears his favorite song is a conditioned response. According to the definition of the conditioned response, Nathan is responding to previously neutral stimuli. That is, his actions are based on his previous actions of checking his phone every time he hears his favorite song playing.

So, the correct answer is conditioned response.