A simple sentence contains only one independent clause and no dependent clauses. Here's the assessment for each sentence:

1. ✓ The Japanese eat healthily.
2. I like gardening because I love plants and flowers. (Not simple; contains two independent clauses joined by "because.")
3. ✓ My family often goes cycling in the countryside.
4. ✓ My sister likes surfing the Internet.
5. ✓ Soft drinks are not good for your health.
6. I like gardening because I love plants and flowers. (Duplicate of #2; also not simple for the same reason.)
7. ✓ Coloured vegetables are good food.
8. My sister doesn’t like horse riding because she’s afraid of horses. (Not simple; it has a dependent clause.)
9. ✓ My favourite outdoor activity is cycling.
10. It is raining, so we stay at home and watch a film. (Not simple; contains two independent clauses joined by "so.")
11. ✓ We need vitamin A for our eyes.
12. My father likes coffee, but I prefer orange juice. (Not simple; contains two independent clauses joined by "but.")
13. ✓ Being active helps keep you fit.
14. ✓ My parents go cycling every Sunday.
15. ✓ The weather may affect our skin condition.