Answer:
a. were
Explanation:
In any given sentence, the subject-verb agreement dictates the verb form of the sentence. And accordingly, the tense of the verb also depends on the subject, whether it is singular or plural.
In the given sentence "Last year, the council members ____ happy to visit their sister city in Argentina", the subject is "council members". And so, depending on this, we can easily deduce that the verb will be in the plural form. As the subject is plural, we will use "were". So, the final sentence will become
"Last year, the council members were happy to visit their sister city in Argentina".
Thus, the correct answer is option a.
