Answer
See answer below.
Work Step by Step
a) $C_2H_4$ is oxidized because carbon's oxidation number goes from -2 to +4, in $CO_2$, thus it's the reducing agent.
$O_2$ is reduced because oxygen's oxidation number goes from 0 to -2, in water and $CO_2$, thus it's the oxidizing agent.
b) $Si$ is oxidized because silicon's oxidation number goes from 0 to +4, in $SiCl_4$, thus it's the reducing agent.
$Cl_2$ is reduced because chlorine's oxidation number goes from 0 to -1, in $SiCl_4$, thus it's the oxidizing agent.
