The correct answer is: C. otherwise
The word “otherwise” is used to introduce a consequence or alternative. In this case, the consequence of not walking fast is that the speaker will miss the bus.
The other options are not as appropriate in this context. The word “but” is used to introduce a contrast. In this case, there is no contrast between walking fast and missing the bus. The word “and” is used to join two things together. In this case, walking fast and missing the bus are not two things that can be joined together. The word “so” is used to introduce a result. In this case, there is no result of walking fast or missing the bus.
Here are some examples of how the word “otherwise” can be used in other sentences:
- You need to study hard, otherwise you will fail the exam.
- I will call you if I change my plans, otherwise I will see you at 8pm.
- You should wear a coat, otherwise you will get cold.