The correct answer is B: Open the client airway.
If a client is unresponsive, the first step the nurse should take is to open the client's airway by tilting the head back and lifting the chin. This is important because it allows the client to breathe properly and helps to prevent respiratory arrest. After opening the airway, the nurse should check for breathing and a pulse. If the client is not breathing, the nurse should begin rescue breathing. If the client does not have a pulse, the nurse should summon the code team and begin chest compressions. These actions are part of the basic life support (BLS) algorithm, which is used to provide initial care to clients experiencing respiratory or cardiac arrest.