According to the Bohr model, the Balmer series corresponds to electronic transitions from higher energy levels ($n_2 > 2$) to the first excited state ($n_1 = 2$). Transitions to the ground state ($n_1 = 1$) constitute the Lyman series . Furthermore, as the principal quantum number $n$ increases, the energy levels become closer together, causing the spectral lines (frequencies) to become closely packed near the series limit . Therefore, statements (b) and (d) are correct.