Which of these redox reactions is spontaneous as written? a. ni + mg2+ → ni2+ + mg b. al + fe3+ → al3+ + fe c. hg + al3+ → hg2+ + al d. zn + k+ → zn2+ + k,

According to Electro-chemical series the metal with more -ve potential will replace the metals below it in series in their salts and tend to oxidize.
A) not occurs since Mg more reactive than Ni (with -ve potential)
B) potential of Al is more -ve than iron so it can replace it in its salt and the reaction can occur spontaneously so B is the correct answer.
C) Mercury cannot replace Al 
D) Zinc cannot also replace  potassium

The ratio is 3: 2. i.e. c.