Option (B) and (D) both are correct.
The complexes which have 17 electrons are act as a oxidizing agent and which have 19 or 20 electrons they act as reducing agent.
Here,
\([Fe(Co)_5] = 8 + 2\times 5 = 18 e^-\)
\([Mn(Co)_5] = 7 + 2 \times 5 = 17 e^-\)
\([Co(NO_2)_6]^{4 -} = 9 + (1\times6)+ 4 = 19e^-\)
\([Co(Co)_4] = 9 + 2\times 4 = 17 e^-\)
Hence, \([Mn(Co)_5] \) and \([Co(Co)_4] \) may act as a oxidizing agent.