The correct answer is (A).
Getting to the Answer:When the underlined portion contains punctuation, check to make sure that the correct punctuation is used. In this question, the comma after "cave" along with the FANBOYS conjunction "and" correctly separates two independent clauses. "I realized" is parenthetical information properly bracketed off by commas. The underlined portion is correct as-is.
(A) Correct: a comma and FANBOYS conjunction ("cave, and") correctly separate two independent clauses, and a pair of commas correctly brackets off the nonessential phrase "I realized"
(B) Incorrect: "I realized" is nonessential information and must be set off by a pair of commas