Better if you guys explain why you eliminated each answer as opposed to stating "correct".
Here:
Australia is
one of the countries that have imposed sanctions on Iran.
(A) one of the countries that
have imposed
sanctions on
A is correct because the finite verb "have", contained within the relative clause, agrees with the subject "countries", and the correct idiom, "sanctions on", is used.
(B) one of the
countries that
has imposed sanctions against
Error 1 - Singular "has" does not agree with plural "countries"
Error 2 - The correct idiom is "sanctions on"
(C) one of the countries
which have imposed sanctions on
Error 1 - Modifier "Which" must be preceded by a preposition or a comma
Error 2 - restrictive 'that' is more sensible as the info contained in the relative clause is essential to the sentence meaning
(D) one of the countries
which has imposed sanctions on
Same as (C)
(E) one of the countries that
have imposed sanctions
againstError 1 - The correct idiom is "sanctions on"