Read the passage: so not only is the rain barrel water free, but it’s also free of chlorine and minerals so it’s really great for your plants and vegetables. which best explains how the passage supports the message: a rain barrel is a small investment for big dollar and environmental benefits?

it explains that water collected in a rain barrel doesn't cost you anything.

it explains that water free of chlorine and minerals is better for plants.

it explains one way rain barrels benefit you, and one way rain barrels benefit plants.

it explains that the water is healthy for both humans and plants.

It explains one way rain barrels benefit you, and one way rain barrels benefit plants.

Explanation:

It benefits you for being free. Not having to pay for something is always good.  The other thing that is said is: "it’s also free of chlorine and minerals so it’s really great for your plants and vegetables".  So as the text makes it clear, it is good for you and the plants.

How about these?  

Explanation:

Telling the truth all the time = honest

If you are honest, you are truthful and don't tell lies.

To have a lot of similar interests = like-minded

Like-minded people have similar interests and opinions.

To depend on someone = rely on

If you rely on someone, you are depending on them.

To have a good relationship = be in harmony

People who have good relationships are in harmony with each other.