What imperative measure should be taken for the safety of exposed cylinder tanks?

When dealing with exposed cylinder tanks in a fire situation, the imperative measure to ensure safety is to cool the tanks by directing water streams onto them. This technique helps to reduce the temperature of the tanks, preventing potential failure or explosion due to overheating. Pressurized gas cylinders can become extremely hazardous when heated, as the pressure inside the cylinder increases, leading to a risk of rupture or venting of the gas.

Cooling the tanks with water helps maintain their structural integrity during the emergency situation, effectively mitigating the risk posed by the heat. Additionally, water streams can also serve to protect surrounding areas from ignition or secondary fires caused by the hot surfaces of the cylinders.

Other options, such as not cooling the tanks or allowing them to ventilate spontaneously, would increase the risk of an explosive event due to the potential buildup of pressure or gas. Replacing tank caps before cooling does not address the immediate danger posed by the heat and can potentially complicate emergency operations. Therefore, directing water streams to cool the tank is the most effective and imperative safety measure.