What must be applied on a sounding tape when sounding a fuel tank?

When sounding a fuel tank, water indicating paste must be applied to the sounding tape because it serves a critical purpose. The paste helps to identify the presence of water at the bottom of the tank. Since water is denser than fuel, it will settle below the fuel layer. When the sounding tape, which is inserted into the tank to measure the depth of the fuel and any potential water, touches the water, the paste changes color or reacts in a specific way, signaling that water is present.

This is important for operational safety and maintenance, as the presence of water can lead to issues such as corrosion, fuel contamination, and reduced efficiency in the fuel system. The other options do not serve this specific function and are unsuitable for accurately determining the presence of water in a fuel tank.