Correct Response: A. The Sun's energy output is maintained by nuclear fusion occurring in the core. Nuclear fusion converts the Sun's most abundant element, hydrogen, into helium, producing energy that then flows outward from the core. This nuclear process occurs at extremely high temperatures through a multistep reaction called the proton-proton chain reaction. The chromosphere (B) is the middle layer of the Sun's atmosphere and while it does transmit energy produced in the core, it does not generate energy from combustion of gases. None of the processes that occur in the corona, the outermost layer of the solar atmosphere (C), are important in maintaining the Sun's energy output. The photosphere (D) is the lowest layer of the solar atmosphere (the Sun's "surface") and does not generate energy from the breakdown of high-energy particles, though it does transmit energy carried outward through the convective zone.
