Final answer:
The most significant factor affecting a fuel sample's conductivity is Temperature, which directly impacts solute solubility and thus the conductivity of solutions.
Step-by-step explanation:
The most significant factor that affects a fuel sample's conductivity reading is Temperature (A). Temperature influences the solubility of substances, including gases, in a solvent. Specifically, as temperature increases, gas solubility typically decreases. This relationship implies that at higher temperatures, the conductivity of a solution would generally be higher due to increased solubility of ionic compounds. Therefore, when speaking about fuel's conductivity, one must account for the temperature conditions under which the reading is taken because they significantly impact the results.
It is important to note that while other factors like pressure (B), salinity (C), and viscosity (D) impact the physical properties of substances, temperature is directly related to the kinetic energy of molecules, affecting how solutes dissolve and consequently the electrical conductivity of the solution.