As an illustration, #3
For a t-shirt design, they're all iffy. The first issue is the big square background. You want to avoid this type of shape since it's going to create a lower quality print. This is because it's going to use more ink and might not feel good to the touch depending on the type of printing used. I would recommend making it some other shape. I think a ripped effect around the edges would be cool. Maybe even just a circle. Maybe cut out a mountain shape to mimic the mountains there.
The second thing is the illustration, particularly the background has gradients in it. From experience, gradients are difficult to work with with t-shirt printing and it will limit the options available to you. So #3 also works better for this, but I would recommend giving the same style to the background as well so it has no gradients.
If you can, make it as a vector, but if you don't know how to do it then don't do it. And do not go to Fiverr to do it (they'll just throw it through an automatic vectorizer thing which anyone can do for free and it will look really bad). Most places probably vectorize nowadays which is another reason to not have gradients. Whatever place you use, you may want to check with them and the best format they accept.
Also if you don't vectorize your image and they don't do it for you, then make sure your image is nice and big so it isn't blurry when printed.
