$\pt$-differential production cross sections of non-prompt $\Dzero$ (top-left), $\Dplus$ (top-right), and $\Ds$ (bottom) mesons at midrapidity, $|y|<0.5$, in pp collisions at $\s=13$ TeV compared with the predictions from FONLL + PYTHIA 8 , GM-VFNS , and TAMU + PYTHIA 8  calculations. The lower panels report the data-to-model ratios. The vertical bars and empty boxes represent the statistical and systematic uncertainties, respectively.