I would say B .Fresh water.
Because we need to have fresh water to survive ,water without the salt, that causes dehydration. So C. would be ruled out. Also fresh water( rain water) is needed to grow crops, without it soil wouldn't be up to par for farming, so that rules out D. Oil, to us is only valuable because of its place in our economy. We need it to fuel vehicles (etc.) Oil is more of a money thing in my opinion so I would rule that out too...for the record this is just off the top of y head. Im not sure if its correct