What is the reducing agent is reaction below?

6AgNO4(aq) + PH3(g) + 3H2O(l) → 6Ag(s) + H3PO3(g) + 6HNO3(aq)

A. HNO3(aq)
B. H2O(l)
C. PH3(g)
D. AgNO3(aq)

Correct Answer:

Option C – PH3(g)