Despite having discussed the matter with all his teachers,
Lewis is still unable to decide if he should pay more attention to understanding the concepts or to their application.
(A) Lewis is still unable to decide
if he should pay more attention to understanding the concepts or to their application -
We need "whether" instead of "if". We use "if" in the case of conditionality and since there is no condition here the usage of "if" is incorrect.(B) Lewis is still confused between paying more attention to understanding the concepts
or to their application -
Correct idiom is "between-and"(C)
it is still difficult for Lewis decide
if he should pay more attention to understanding the concepts or to their application -
We need the noun "Lewis"(D)
Lewis is still not able to decide
whether he should pay more attention to understanding the concepts or to their application
(E)
there is still confusion in Lewis’ mind with regards to understanding the concepts or to their application -
Same as C