Supermassive black holes (SMBHs) co-evolve with their host galaxies, but the broad-line region (BLR) and accretion disc lie far below current spatial resolution. Reverberation mapping (RM) links continuum and line variability to measure BLR sizes and calibrate the R_BLR–L relation used for single-epoch mass estimates, yet this calibration is weakest for faint intermediate-mass black holes (IMBHs) and bright jet-dominated quasars. This thesis combines focused RM campaigns, jet–disc decomposition in radio-loud AGNs, large-sample optical variability analysis, and pilot work for geometric distance measurements to address these limitations.

At low luminosities, a two-night spectrophotometric RM campaign on the IMBH AGN NGC 4395 with the 1.3-m DFOT and 3.6-m DOT yields an Hα lag of ≈125 light-minutes and M_BH ≈ 2.2 × 10^4 M_sun, confirming it lies well below the canonical R_BLR–L relation. At the high-luminosity, radio-loud end, Steward data for PKS 0736+017 give an Hβ lag of ≈66 days and M_BH ≈ 7.3 × 10^7 M_sun. To study the effect of non-thermal emission in the size–luminosity diagram, we use multi-epoch photometry, polarimetry, and 15-GHz OVRO light curves for PKS 0736+017 and PKS 1510–089 and show that PKS 0736+017 is jet-dominated (disc fraction ≈40%, jet-corrected Hβ lag ≈89 days, radio lag ≈399 days), whereas PKS 1510–089 is disc-dominated (disc fraction ≈74%, Hβ lag ≈48 days, radio lag ≈101 days), quantifying whether jets bias lags, luminosities, and R_BLR–L_5100 placements.

To investigate what drives AGN optical variability, I model multi-year ZTF g-band light curves of SDSS DR14 quasars with a Damped Random Walk, finding strong anti-correlations of variability amplitudes with bolometric luminosity(L_bol) and Eddington ratio(lambda_Edd), but only shallow, weak trends of the DRW timescale with black hole mass(M_BH), L_bol, and lambda_Edd, both globally and in redshift bins, consistent with predominantly disc-driven stochastic variability. Finally, weekly DOT+HCT spectrophotometry of the SARM targets Mrk 1048 and Mrk 618, combined with ASAS-SN/ZTF light curves, yields new Hβ/Hα lags and RM-based masses of 6.3^(+2.0)(-2.1) × 10^7 M_sun and 1.2^(+0.4)(-0.6) × 10^7 M_sun, respectively, as a first step toward AGN-based geometric distance indicators and an independent handle on the current H_0 tension.