Final answer:
The more reasonable measurement for the distance between the tracks on a railroad is 1.435 x 10^3 mm, which corresponds to the standard railroad track gauge of 1.435 meters.
Step-by-step explanation:
When determining the reasonable measurement for the distance between the tracks on a railroad, we need to consider the typical dimensions of such a structure. Railroad track gauges (the distance between the rails) are generally standardized to ensure the safe and efficient movement of trains. The most widely used standard gauge is 1.435 meters, which would be 1.435 x 103 mm since there are 1,000 millimeters in a meter.
Option A, 1.435 x 10-3 mm, would be far too small, as this converts to 0.001435 mm, which is not practical for the width of a railroad track. Therefore, the more reasonable measurement for the distance between the tracks on a railroad is option B, 1.435 x 103 mm.