Second letter U
Kind of a lengthy explanation..but here goes:
GAMETE showed up a 0
CORNEA showed up a 0
So GUILTY
GUINEA
GULLEY
GUILTY showed 2 so possible letters: IL, UL, IY, UY, LY, UI
LY is not possible since WIELDY shows up 1
UI is not possible since GUINEA shows up 1
So its IL, UL, IY or UY
In any case I or U are correct
If IL was correct, consider GUILTY and GULLEY. Out of the non striked letters only I is replaced with L but it still showed up 2. So IL is out. With the same logic IY is out too.
So its down to UL or UY.
In any case U is correct.