Which of these safety signs is the caution sign indicating traffic passing on both sides?

Answer: ② Ⓑ

Ⓑ, the red-bordered triangle with an arrow splitting to both sides, is the caution sign for passing on both sides (No. 122). Ⓐ is the two-way traffic caution sign, while the round blue signs Ⓒ and Ⓓ are instruction signs (turn left or right, and pass on both sides). [Enforcement Rule of the Road Traffic Act, Annex 6]