Here are the corrections for each sentence:

1. **The more** pollution there is in the air, **the harder** it is to breathe.
- Correction: **A. The more** (should be "As more" or rephrased)

2. **The early** you leave for school, **the less** traffic you will encounter.
- Correction: **A. early** (should be "The earlier")

3. We will go to the park for a picnic if the weather **will be** nice tomorrow.
- Correction: **D. will be** (should be "is")

4. It is important for children to be aware of when **crossing** the busy street safely.
- Correction: **C. crossing** (should be "to cross")

5. **The taller** the building are, **the better** the view from the top becomes.
- Correction: **B. the building** (should be "the buildings")

So, the underlined parts that need correction are A (1), A (2), D (3), C (4), and B (5).