Which of the following sentences is correctly punctuated?

The sentence "It's raining, so we stayed indoors." is correctly punctuated because it properly uses the contraction "it's," which stands for "it is." The use of a comma before the coordinating conjunction "so" is also correct, as it separates two independent clauses. This punctuation indicates a natural pause in the sentence, clarifying the relationship between the two clauses. This construction effectively conveys that the reason for staying indoors is the rain.

The other options either use "its" incorrectly (which is a possessive form) instead of the contraction "it's," or lack proper punctuation, which compromises clarity. Understanding the difference between "it's" and "its" is crucial for proper sentence construction.