Answer: dem
Step-by-step explanation:
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.
In the dative case,
der turns to dem,
das turns to dem,
die turns to der,
and die (in the plural sense) turns to den.
Hope this helps you!