The correct answer is A) a sponge used to mop up and store condensation must be kept salt free. The reason this is the correct answer is that when collecting condensation for drinking water, it is important to avoid any contamination, including from salt. A sponge used to collect the condensation must be kept salt-free to prevent the water from becoming saline, which would make it undrinkable. The other options are incorrect because they do not address the importance of preventing salt contamination, which is a key consideration when collecting drinking water from condensation.