Correct answer: (a) 3

Explanation:

The reaction given is

Cr₂O₇^2- + Fe²⁺ + C₂O₄^2- → Cr³⁺ + Fe³⁺ + CO₂

Cr₂O₇^2- → 2Cr³⁺

On balancing

14H⁺ + Cr₂O₇^2- + 6e^- → 2Cr³⁺+ 7H₂O  ......(i)

Fe²⁺ → Fe³⁺ + e^- ..........(ii)

C₂O₄^2- → CO₂ + e^- .........(iii)

On adding all the three equations.

Cr₂O₇^2- + Fe²⁺ + C₂O₄^2- + 14H⁺ + 3e^-→ 2Cr³⁺ + Fe³⁺ + 2CO₂ + 7H₂O

Hence the total no. of electrons involved in the reaction = 3