At the official function organised for the attending dignitaries, guests arrived in scrumptious attire for the banquet.

'Scrumptious' means extremely tasty or delicious, used for food, not attire. For attire at an official banquet, 'sumptuous' is appropriate, meaning splendid, expensive-looking, and luxurious. 'Synergistic' relates to combined effects being greater than the sum of individual effects. 'Stentorian' refers to a loud, powerful voice. 'Scrimpy' means meagre or scanty. Therefore, 'sumptuous' correctly improves the sentence in the context of grand banquet attire.