💡 Detailed Explanation
The correct choice is Option 3: 'Put the worn socks into the washing machine.' The mother initially orders him to hang up the clothes ('床の上にある服を...'). However, when the son mentions the dirty socks ('脱ぎっぱなしの靴下がここにあるよ'), the mother overrides the order: 'じゃあ、それを先に洗濯機に入れてきちゃって。そのあと服をやってね' (Well, put them in the washing machine first. Do the clothes after that). Thus, the very first action he takes is taking the socks to the washing machine.