To solve the problem of how many people can ride the roller coaster at once, you need to know how many seats are available in each car.
The fact you need is:
"There are three rows of seats in each car."
Additionally, you would need to know how many seats are in each row to calculate the total capacity. However, based on the options given, the number of rows is the most relevant fact provided. The other options do not contribute to solving the problem of determining how many people can ride at once.