Explanation:

- Option 1: "are" is used for plural subjects. "A big park" is singular, so it doesn't fit here.

- Option 2: "may be" suggests possibility or uncertainty. It's correct in context where the park's presence is uncertain, but doesn't really fit the sentence provided, which seems to describe a known fact.

- Option 3: "is" is correct. "Is" is used for singular subjects in present tense, which matches "a big park" and indicates a fact.

- Option 4: "were" is past tense and plural, making it incorrect for this context.