Hi, the answer is dem. This is because when you use 'unter' in German, it causes the article next to it to be in the dative cause. The word 'Fenster' in German means window, and it is 'das Fenster'; its gender is neuter.
It says bring them in the right order and then write the letter correctly. You want to have a week of ski trip at the end of February. You have four children, 5 to 13 y/o you need a double room and a four bed room and halfpesion