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We have measured the γ -ray line production cross sections in proton-induced nuclear reactions on various target nuclei (12C, 16O, 24Mg, 28Si, 56Fe) of chemical elements abundant in astrophysical sites (solar flares, the interstellar medium, cosmic compact objects) over the incident energy range of E p = 30 – 200 MeV. We carried out experimental campaigns in joint collaboration at the K = 200 separated sector cyclotron of iThemba LABS using a high-energy resolution, high-efficiency detection array composed of 8 Compton-suppressed clover detectors comprising 32 HP-Ge crystals for recording the γ -ray energy spectra. In the current paper, we focus on γ -ray de-excitation lines produced in proton irradiations of n a t C and Mylar targets, in particular, on the prominent 4.439 and 6.129 MeV lines of 12C and 16O which are among the strongest lines emitted in solar flares and in interactions of low-energy cosmic rays (LECRs) with the gas and dust of the inner galaxy. We report new γ -ray production experimental cross section data for ten nuclear γ -ray lines that we compare to previous low-energy data sets from the literature, to the predictions of the TALYS code of nuclear reactions and to a semi-empirical compilation. In the first approach, performing calculations with default input parameters of TALYS we observed substantial deviations between the predicted cross sections and experimental data. Then, using modified optical model potential (OMP) and nuclear level deformation parameters as input data we generated theoretical excitation functions for the above two main lines fully consistent with experimental data. In contrast, the experimental data sets for the other eight analyzed lines from the two proton-irradiated targets exhibit significant deviations with the predicted cross section values. We also report line-shape experimental data for the line complex observed at E γ = 4.44 MeV in irradiations of the two targets. Finally, we emphasize the astrophysical implications of our results. [ABSTRACT FROM AUTHOR]