Final answer:
A convertible with an adult and two children and a minivan with two adults may use the freeway on-ramp marked with the white sign 'carpools 3 or more only'
Step-by-step explanation:
The vehicles that may use the freeway on-ramp marked with the white sign 'carpools 3 or more only' are A) A convertible with an adult and two children and C) A minivan with two adults. This is because both these options have three or more people in the vehicle, which meets the carpool requirement. The white sign indicating "carpools 3 or more only" typically designates a carpool lane, and vehicles with three or more occupants are allowed to use that lane. In this scenario, a minivan with two adults satisfies the requirement for a carpool.