Researchers hypothesize that granitic soil is the ideal construction material for the desert tortoise because it is not
so hard that it makes burrowing difficult or so soft that it could cause tunnels to collapse.
Option Elimination -
(A) so hard that it makes burrowing difficult or so soft that it could cause - use the correct cause and effect. So "hard" (cause) that "it makes burrowing difficult" (effect). "it" refers to soil.
(B) hard enough to make burrowing difficult or soft enough as to cause - "enough to" is used to show adequacy. E.g., Sarah studied hard enough to earn a scholarship. But it is not used for cause and effect. Moreover, "enough as to" after "or" is wrong.
(C) so hard as to make burrowing difficult or soft enough so it causes - the usage of " so (cause) as to (effect) is not parallel.
(D) as hard as to make burrowing difficult or as soft as to cause - there is no comparison here. "as X as" is used for comparison. E.g., She is as tall as her sister. There is no comparison here.
(E) too hard, making burrowing difficult, nor too soft, so as to cause - "so as to" means "in order to" E.g., She arrived early so as to secure a good seat for the presentation. The construction ("nor too soft" in order to cause to tunnels to collapse) is a total mess.