While most ecologists believe that the Striped Kangaroo of southern Australia should be added to the Endangered Species List, the World Conservation Union has stated that the kangaroo will be listed
only if it could be proved to be a distinctly different species from other kangaroos.
The first difference between the options is the position of "only". The kangaroo will be added "ONLY if it is proved/if it is ONLY proved". The word only does not refer to the verb: it states a condition and should be placed if front of the condition itself. If I place "only" before the verb,it will modify the verb. Out C and E.
Example: I will be part of the team only if I jump higher. Correct VS I will be part of the team if I only jump higher. Wrong
D if it was proved to be
only a distinct and different species
Only is also misplaced here: "proved to be only a dinstinct (...)" only here refers to "distinctly different species", and it's clearly wrong.
A and B remain
A only if it could be proved to be a distinctly different species
B only if it is proved to be a different species, distinct
First problem with A: "
could", the kangaroo will be listed only if it
could, we are talking about a necessary condition so "could" should be replaced by "is". Second problem with A: "
distinctly ", it modifies a noun so must be an adj (not an adverb).
B solves those two problems.