Let's evaluate each option:
(A) "on the basis of avoidance of expanded executive power alone"
Idiomatically, "on the grounds of" is the more common phrase to express a reason or justification, and "avoiding" is simpler and clearer than "avoidance of".
I think this is not the most concise option.
(can be eliminated, but let's keep)
(B) "on the grounds of avoiding expanded executive power alone"
The phrase "on the grounds of" is more idiomatic. I think there are no other issues.
(keep this)
(C) "solely in that, the expansion of executive power should be avoided"
The phrase "solely in that" is not idiomatic or coherent in this context. Also this option sounds ungrammatical.
Also, I think, the comma after "in that" is not necessary and disrupts the flow of the sentence. It is creating a break. "In that" is a conjunction that is used to connect two clauses, so there should not be a comma separating the two clauses it is connecting.
(eliminate)
(D) "if only because the expansion of executive power should be avoided"
This seems to be grammatically correct and maintains clarity in delivering the reason for the decision being poor.
The phrase "if only because" sets up a conditional clause that provides a reason or justification for the main clause.
(keep this)
(E) "while the expansion of executive power should be avoided"
The conjunction "while" is not used correctly here. It implies a contrast or simultaneous event.
The intended meaning is getting changed.
(eliminate)
On re evaluating, we can eliminate option A as well.
Now, between B and D,
both are grammatically correct.
However in option B, the word "alone" at the end may cause some ambiguity as it could refer either to the Supreme Court's sole reason or to the sole action of avoiding expanded power.
So I will go with D.