They were not starving during the flight because they a meal earlier in one of the airport restaurants.

Исходя из контекста, нужно вставить глагол в форме Past Perfect, чтобы показать, что действие (приём пищи) произошло до того, как они перестали голодать.

They were not starving during the flight because they had had a meal earlier in one of the airport restaurants.

Ответ: had had