选项：b.on their bikes
解析：乘坐交通工具有三种讲法：
1.by + 不带冠词的单数交通工具：前面不可以加冠词或物主代词,用作方式状语.如：
Many of the students in my class go to school by bike.
They go to work by bus.
2.on + 单数或复数交通工具：前面必须加冠词或物主代词,也用作方式状语.
Many of the students in my class go to school on their buses.
They go to work on the bus.
3.take + 单数或复数交通工具：前面可以加冠词或物主代词,用作谓语动词.
Many of the students in my class take their buikes to go to school.
They take buses to go to work.
He takes the car to go to work.