The core of this argument is:
modern capitalism requires big corporations -> therefore modern capitalism promotes communal ties
So we need to pick an assumption that supports this conclusion. I think here many readers will also even formulate an idea of what an assumption could look like before even getting to the answer choices. We probably want to find something that connects big corporations to communal ties (remember, we leave our outside knowledge behind, in case anyone finds corporate culture less than communal ).
With that in mind, let's check out the answers:
(A) is out of scope. We don't care about other economic systems.
(B) is also out of scope. We don't care about the purpose of modern capitalism - we just care about what it does. These are not the same thing unless you are bringing in your own assumptions about the real word
(C) is also out of scope. We don't know or care about corporations that compete with one another.
(D) BINGO! This is exactly what we predicted up above.
(E) is out of scope for the same reason as (A).