Here are the suggested answers for the blanks in the passage:

1. **C**. of (indicating being part of the country)
2. **A**. bigger (comparing Da Lat and Hanoi)
3. **A**. visited (referring to visiting places)
4. **D**. than (used for comparison of weather)

So, the completed passage would read:

Dear Linda,
I’m back Da Lat, a city in the centre **(1) C. of** Vietnam. Let me tell you about the wonderful time I had there with my dad and my mum last week. Da Lat is a beautiful city. It’s **(2) A. bigger** than Hanoi. It’s more peaceful, too. We **(3) A. visited** some interesting places such as Langbiang Peak, Prenn Waterfall, Valley of Love and Truc Lam Monastery. The weather was fine. In Da Lat, it was cooler and drier **(4) D. than** in Hanoi. The food was delicious and cheap. But the clothes were more expensive.

We stayed there for five days before coming back to Ha Noi.
Bye,
Nam