What should you do when turning left at an intersection where there are no traffic signs or signals?

Answer: B — Yield to any vehicle approaching from the opposite direction

When turning left at an intersection with no traffic signs or signals, it is important to always yield to any vehicles approaching from the opposite direction. This is essential for ensuring the safety of both yourself and other drivers. Option A is incorrect because simply slowing down may not give you enough time to yield to oncoming vehicles. Option C is incorrect because increasing speed is never advisable when approaching an intersection. Option D is incorrect because sounding your horn is meant for warning others of your presence, not as a substitute for yielding at an intersection.