Final answer:
The only mode of intercity travel that has remained relatively constant since about 1950 is road transportation.
Step-by-step explanation:
The only mode of intercity travel which has remained relatively constant since about 1950 is road transportation (c).
While rail transportation (a) and water transportation (d) have seen changes and advancements over time, road transportation has remained largely consistent. Air transportation (b) has significantly expanded since 1950, with the rise of commercial airlines and increased accessibility to air travel.
Keywords: intercity travel, road transportation, rail transportation, water transportation, air transportation