I try to negate by finding the weakest possible scenario in which the answer chioce is not true. For example, we would negate the statement "all mice have tails" by saying "there is one mouse that has no tail."
We could negate (A) by saying "Not all top-loaders use the same amount of water" or "Some toploaders use different amounts of water"
We could negate (B) by saying "A frontloader detergent does not necessarily dissolve more readily in them than in toploaders"
We could negate (C) by saying "A machine can get clothes clean even with a detergent not specially designed for it"
An unless statement is usually tricky, but we could negate (D) by saying "A detergent can get clothes clean even if it doesn't readily dissolve"
We could negate (E) by saying "Washers that use more water do not always get clothes cleaner than those that use less.