The correct answer is C: This type of tower must not be climbed unless it is retracted, or mechanical safety locking devices have been installed. An important safety rule for crank-up towers is that they must not be climbed unless retracted or mechanical safety locking devices are installed. Crank-up towers can collapse if the mechanism fails while someone is climbing. The tower must be fully retracted (safest) or have mechanical locks engaged to prevent collapse. For amateur radio operators, this is a critical safety requirement that prevents serious injury or death.