Two independent clauses are clauses that can each stand on their own as a complete sentence. When a writer wants to join two such clauses in a single sentence without using a coordinating conjunction like and, but, or so, the correct punctuation mark is a semi-colon.
A comma alone cannot join two independent clauses. Doing that creates a comma splice, which is treated as a grammar error in formal writing. A period does not connect the clauses at all, it separates them into two distinct sentences, so it does not satisfy the requirement of connecting them. A colon is used to introduce a list, an explanation, or an example that follows logically from the first clause, not simply to link two clauses of equal weight and independent meaning.
Only the semi-colon lets two complete, related thoughts sit inside one sentence without a connecting word, which is why option 2 is correct.