I have a question.
which word is suitable for the blank?
Spending time at my grandfather's house was a ( ) event; we visited twice every year for summer and winter vacations.
I think "semiannual" means twice a year, and so does "biannual".
I know what they're looking for, but 4) also works.
Then "semiannual", "biannual" and "delightful" are suitable, aren't they?
I would use "biannual" to mean every two years. Not twice a year.
'Biannual' is twice a year, 'biennial' every two years. Lots of people get them wrong.