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A)  The oxidation state of Oxygen is reduced, from 0 to -1.  So Ammonia is reducing agent.  Nitrogen and hhydrogen are oxidized to a state of +2 and +1 respectively.

b)  Flourine is reduced to floride ion.. oxidation state of -1.
     Water is the reducing agent..
c)  Iron is reduced from an oxidation state of +3 to  0.. Carbon is oxidized from +2 state to +4.  so carbon monozide is reducing agent.

d)  Oxygen is reduced from a state of 0 to -2... hydrogen is oxidized.  so hydrogen is the reducing agent.

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(a) NH3 is a reducing agent.
(b) H2O  is a reducing agent.
(c) CO  is a reducing agent.
(d) H2   is a reducing agent.
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