The phrase "a big fat big back" doesn't inherently convey a clear positive or negative quality, as it largely depends on context and personal perspective. However, if you are asking for a choice between "ugly" and "pretty," it would typically lean towards "ugly" due to the use of descriptors like "big" and "fat," which can carry negative connotations.
So the answer could be: a. ugly
But it's important to remember that beauty is subjective, and different people may have different opinions.