• Driving Force for Tubular Fluid Flow: Glomerular ultrafiltrate is transported down the nephron from Bowman's space to the renal pelvis primarily by a Hydrostatic Pressure Gradient.
• Pressure Dynamics: Glomerular filtration establishes a high hydrostatic pressure in Bowman's capsule ($\sim 15\text{--}20\text{ mmHg}$), whereas the renal pelvis maintains a near-zero pressure ($\sim 0\text{--}2\text{ mmHg}$), propelling fluid down the tubular network without ciliary propulsion or active peristalsis (Option 3).

Correct option: (3) Hydrostatic pressure gradient