Explanation: The characteristic impedance (also known as surge impedance) of a non-resonant transmission line is a fundamental property determined by its physical construction, specifically the inductance and capacitance per unit length. 1. **Diameter of the conductor:** This physical dimension directly influences the line's inductance and capacitance. A larger conductor generally decreases inductance and increases capacitance. 2. **Space between conductors:** The distance separating the conductors significantly affects both inductance and capacitance. Wider spacing typically leads to higher inductance and lower capacitance. 3. **Dielectric constant of insulating material:** The insulating material between the conductors has a dielectric constant, which directly impacts the capacitance per unit length. A higher dielectric constant results in higher capacitance. These three factors are precisely what determine the line's inductance (L) and capacitance (C) per unit length, which in turn define the characteristic impedance ($Z_0 = \sqrt{L/C}$ for a lossless line). Options B and C include factors like "voltage on RF line," which does not determine the characteristic impedance, or use ambiguous terminology, making them incorrect.