Diagnostic plots for selecting radio-AGN using LoTSS data. Red points mark the sources classified as radio-AGN after combining all four diagnostics, and grey points show the star-forming/radio-quiet AGN: (a) D_n(4000) vs L_1.4 GHz/M_⋆ plot. The solid and dashed curves mark the radio-AGN, intermediate and SF/RQAGN division from Best & Heckman (2012) and Sabater et al. (2019); (b) L_Hα versus L_1.4 GHz plot, with the separation lines from Sabater et al. (2019); (c) BPT diagram where the solid curve shows the semi-empirical relation from Kauffmann et al. (2003), and the dashed curve shows the maximum starburst curve from Kewley et al. (2006); (d) WISE colour-colour plot for sources with the division line from Sabater et al. (2019).