"der" -> Dative feminine, after "unter". "Unter" is a changeable preposition that can take either the accusative or dative case, depending on the situation. While it gets more difficult with idiomatic phrases, an easy way to remember for situations like the one above is "DRAM" - dative rest, accusative motion.

I believe it would be "...den Hosen"

Hosen are plural of Hose. In the dative, DER/DAS becomes DEM, Die (feminine) becomes DER, and DIE (plural) becomes DEN.