Here are the best answers to complete each sentence:

1. **When you travel to big cities in Europe, watch out for** **(a)** **pickpocket** and keep your belongings close to you.
2. **One of the main reasons for air pollution is** **(b)** **oil**.
3. **We were late for our flight to Madrid because we got** **(c)** **stuck** in an awful traffic jam.
4. **Come to the fishing market in the morning, and you can see how** **(d)** **bustling** it is.
5. **The railway station was criticized for its lack of disabled** **(a)** **facilities**.
6. **The library also offers private learning** **(c)** **schedule** for students who buy membership.
7. **Tan Son Nhat airport is heavily** **(d)** **congested** during public holidays.

So, the answers are: 1-a, 2-b, 3-c, 4-d, 5-a, 6-c, 7-d.