When stopping at a stop sign, you should

Answer: B — come to a full stop

When stopping at a stop sign, it is important to come to a full stop before proceeding. Option A, sounding your horn first, is incorrect because it is not necessary and could be considered disruptive to other drivers. Option C, stopping and then proceeding, is incorrect because it does not specify coming to a full stop. Option D, slowing down and proceeding, is incorrect because it does not ensure that the vehicle is completely stopped, which is crucial for ensuring the safety of all drivers and pedestrians. Therefore, the correct answer is B, coming to a full stop before proceeding.