(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

(C) Incorrect: a semicolon separates two independent clauses; however, in this placement, the semicolon incorrectly separates the subject of the second clause, "bats," from its verb, "can find"

(D) Incorrect: "I realized" is nonessential information and must be set off by a pair of commas