The best rule for assumptions is to negate the given options and see whether the argument falls apart.
Negate E and you get: rice is NOT the only crop available for which supply&demand is balanced.
This is out of scope. We are not interested in what happens to other crops. To the extreme, this also strengthens the argument because if rice supply&demand is balanced, then the author's assertion that the production shortage and population increase will make Teruvia import rice will be true.
I'll give you an example of a simple assumption question:
My poodle is intelligent because all dogs are intelligent
What did we assume in the above for the argument to hold? We assumed that poodles are dogs. If poodles were not dogs, then the argument that "my poodle is intelligent" falls apart.