The correct answer to this open question is the following.
These are the three different analogies.
1.- "If people donate blood you are probably not only saving a human's life but an entire generation."
I think this sentence is good and could work because the meaning is that if every single one of us donates blood, we can help give life hope to thousand or millions of people who can breed the next generation on Earth.
2.- "If you donate blood is like mother nature sharing its waters to grow plant and threes. She gives them life."
This analogy has to do with how mother nature takes care of every living sentient on the planet, the way we could take care of other people in need, by donating blood.
3.- "Donating blood is like the Sun shining its rays of light over the Earth. It guarantees life for all."
This analogy tries to create awareness in that we could be "the sun" for other people who needs or "rays of life" or our living hope by donating blood.