Let us look at the answer options -
A - the usage of comma before be - "could, be" is incorrect.
B - "but" requires parallelism. "but that" is not parallel to anything.
C - The usage of "would" is incorrect.
Would is used under 2 circumstances -
1. If we want to talk about the future from the past's point of view
Ex - In 2014, people predicted that Modi would win the elections. (In that time frame, elections= in the future).
2. Things that are not true or unlikely to happen.
Ex - If I were rich, I would donate money to charity. (I am not rich)
If she came to the meeting tomorrow, she would bring her laptop. (it is unlikely that she will attend the meeting tomorrow).
None of these usages are correct in option C. When you make a speculate about the future, use 'will'.
for ex - I think Hillary will win this year's presidential election.
D - 'believe that' is the correct construction.
E - correct answer.
Note the use of could.
If I were rich, I would donate money to charity. (I will definitely donate money to charity if I somehow get rich).
If I were rich, I could donate money to charity. (I will be able to but I may or may not donate money).