The correct option is D 6
For salts the n-factor is
Total positive charge or ∥Total negative charge ∥
Fe2(HPO4)3⇌2Fe3++3 HPO2−4
Charge of Fe3+=+3
Total 2 Fe atoms is present.
n-factor =2×3=6
Alternative way :
Charge of HPO2−4=−2
Total 3 HPO2−4 atoms are present.
n-factor =∥−2∥×3=6
So , n-factor of Fe3+,HPO2−4 is 6.