creating a selection list out of variable criteria

These will all require array formula, so be careful as you edit them, they can be tricky.

A17: =MAX(IF($C$2:$J$9="x", $C$1:$J$1, 0))
...confirmed by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER. You will know the array is active when you see curly braces { } appear around your formula. If you do not CTRL+SHIFT+ENTER you will get an error or a clearly incorrect answer. Press F2 on that cell and try again.

A21: =IFERROR(INDEX(A$2:A$9, SMALL(IF(OFFSET($B$2:$B$9, , MATCH($A$17, $C$1:$J$1, 0), , )="x", ROW(A$2:A$9)-1, ""), ROW(A1))), "")
(array entered)

Now copy A21 down and across that small table.