Answer
See answer below.
Work Step by Step
a) $Au$ has been oxidized, since its oxidation number increases from 0 to +1, thus it's the reducing agent.
$O_2$ has been reduced, since its oxidation number decreases from 0 to -1, thus it's the oxidizing agent.
b) Number of moles of gold:
$10^6\ g\times 0.019/100\div 196.97\ g/mol=0.9646\ mol$
From stoichiometry: $1.929\ mol$ NaCN
Volume: $1.929\ mol\div 0.075\ M=25.72\ L$
