Sentence with Correct Punctuation:
c) It's a sunny day.

Explanation:

1. Punctuation:
Punctuation is the use of special marks or symbols to help convey meaning and clarify the structure of written language. It helps to indicate pauses, emphasize certain words or phrases, and separate different parts of a sentence.

2. Correct Punctuation:
The correct punctuation in the given options is in option 'c) It's a sunny day'. Let's break down the punctuation in this sentence:

- It's: The contraction "it's" is used to combine the words "it" and "is". The apostrophe (') in "it's" indicates that a letter has been omitted (the 'i' in "is"). This is the correct way to write the contraction for "it is".

- a: The word "a" is an article used before a noun to indicate that the noun is nonspecific or indefinite. It is correctly placed after the apostrophe and before the noun "sunny day".

- sunny day: The phrase "sunny day" refers to weather conditions and describes the type of day. It is correctly written without any punctuation marks.

3. Incorrect Punctuation in Other Options:
Let's analyze the punctuation in the other options to understand why they are incorrect:

- a) She likes to swim: This sentence lacks a punctuation mark at the end. It should end with a period (.) to indicate the completion of the statement.

- b) The dog is brown: This sentence is correctly punctuated. However, it is not the correct answer as the question asks for the sentence with correct punctuation.

- c) It's a sunny day: This sentence is correctly punctuated and is the correct answer.

- d) The cat is sleeping: This sentence is correctly punctuated. However, it is not the correct answer as the question asks for the sentence with correct punctuation.

Conclusion:
The sentence with correct punctuation is option 'c) It's a sunny day'. The apostrophe in "it's" correctly indicates the contraction of "it is", and the sentence is properly punctuated without any errors.