Answer
The boat had the fastest average speed.
Work Step by Step
In order to figure out the average speed, we divide the distance traveled by the time it took:
Car: $360 \div 6 = 60 km/hr$
Boat: $ 375 \div 5= 75 km/hr$
Train: $400 \div 8 =50 km/hr$
The boat had the fastest average speed.