All forms of life on earth are linked together in relationships involving
water
food
the environment
energy

Do you have to choose only one answer? I would say that all of them are true, for example food and energy are the same thing in the way, because food is the source of energy. Similarly all life on earth needs water and water used by some organisms is recycled by others. However I think that the best answer is food since this is the one thing that links all forms of life more directly. For example water can be taken not only from other life forms but also from the environment.