"A" does seem to be correct answer! :)
They held a dinner party to mark the occasion of their fortieth wedding anniversary.
Which is the closest sentence to one given above?
A) The dinner party they gave was to commemorate forty years of marriage.
B) They have been married for forty years and the dinner party is by way of celebration.
C) When they have been married for forty years, they will give a dinner party to celebrate the fact.
D) At the dinner party, everyone congratulated them on forty years of marriage.
E) They have been married for forty years and a dinner party was held in their honour.
I think A, you?