Good Question.. I would have selected 'B'. Would definitely like to know what is wrong with 'B'..

B is wrong because economical is not parrallel with political; economic is the correct adjective. Furthermore, the subject of the sentence for B is "oil", which is incorrect - it's the finite-status of the oil that has become a consideration, not the oil.

"That the supply of oil is finite" is the best answer because "That" (referring to the status of the oil) is the correct subject of the sentence. The sentence in the active voice, so it is preferable over D (which is also correct, but just in the passive voice).

Thanks for the explanation!!

but why not C? in C economical is parallel to political, whereas economic would should be parallel to politic

Economicial is not parallel with policial. The correct form is economic. Economical means "thrifty", not "relating to economics".

Just because the words sound alike, does not make them parallel.

Option A.
First of all, the use of "economical;' is not parallel with political has economical in the context means frugal. So it does not follow the pattern of the entire sentence.

Option B.
This option suffers the same fate as option A. As it does not fully express the right contextual meaning of the pattern of the sentence.

Option C.
We shall be left with option D and E because this option suffers the same fate as the two options above, and therefore it is incorrect.

Option D.
With critical consideration we can see that this option is incorrect, Option E give a better analysis on why 'oil has become an economic and political consideration'
Article 'the' does not give an introduction to why this will be so.

Option E.
This option clearly states why the supply of oil has become an economic and political consideration.
It is important to know that the conjunction 'that' is used introducing a clause or a subordinate clause expressing a reason or a clause. This option contains the right properties, therefore, this is the correct option.