Final answer:
The length restriction for a tractor-trailer combination is not definitively given in the information provided, and it is essential to check local regulations as this can vary by jurisdiction.
Step-by-step explanation:
The length restriction for a tractor-trailer combination varies by jurisdiction, but common international limits are not mentioned in the information provided. To ensure accurate adherence to local regulations, it is necessary to consult specific national or regional laws concerning vehicle dimensions. However, the information provided does not include a definitive limit to address the options given in the question (15 meters, 18 meters, 20 meters, or 25 meters). Considering typical tractor-trailer dimensions, a combination could exceed the default options depending on the type of trailer and the configuration of the truck. It is always important for trucking operations to be familiar with and comply with the relevant vehicle size regulations to ensure safe and legal transportation.