Answer
(a) 1
(b) 2
(c) 1
(d) 3
Work Step by Step
(a) In one mole of FeO, we have 1 mole of iron.
(b) In Fe$_2$O$_3$, we have 2 moles of iron.
(c) In Fe(OH)$_3$, we have 1 mole of iron.
(d) In Fe$_3$O$_4$, we have 3 moles of iron.