The complex having the highest spin-only magnetic moment is:

Step-by-step solution

Spin only magnetic moment is given by μ = n ( n + 2 ) . So, the magnetic moment is directly proportional to the number of unpaired electrons. Fe 3 + have n = 1 ( 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 5 ) . Fe 2 + have n = 4 ( 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 6 ) . Mn 2 + have n = 5 ( 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 5 ) . Ni 2 + have n = 2 ( 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 8 ) .