OPTION A - INCORRECT
This option is wrong because it uses all strive more than each other,. Thus, this makes it incorrect.
OPTION B - INCORRECT
All strive more successfully to - This option resolves error of A, but placement of more successfully is incorrect here.
OPTION C - INCORRECT
Here, we look out for the vital areas and it depends totally on the antecedent of a pronoun. If it's singular, then we will use "its" and if it is plural, we will use "theirs".
So, in this case, EACH is correct in usage and will have to be complemented with 'ITS' and not 'THEIRS'.
OPTION D - CORRECT
This option resolves both errors of option A and B. Aside that, the comparison is for one company. Check and see that 'each' is a singular determiner and it should attract a singular pronoun which is its
OPTION E - INCORRECT
Each is a singular determiner having note that "than another" is an improper phrase that should not follow the determiner.