The correct answer is A) a sponge used to mop up and store condensation must be kept salt free. When collecting condensation for drinking water, it is important to ensure that any sponge or material used to collect the condensation is kept salt-free. This is because saltwater condensation can be harmful if consumed, as the salt content can dehydrate the body further. Straining the condensation through a finely woven cloth, as mentioned in option C, is also a good practice to remove any impurities. However, the key point is that the collection device itself must be maintained in a salt-free condition. The other options are incorrect because: B) only collecting condensation from the bottom of the canopy may not provide enough water, C) while straining is important, it does not address the need to keep the collection device salt-free, and D) chlorine tablets are not necessary for purifying condensation, which is generally clean and safe to drink.