What letter should replace the question mark in the last group of letters? MCJ VPF YKN SA?

Question

Complete the square with the letters of ZEBRA so that no row, column or diagonal line of any length contains the same letter more than once.

What letter must replace the question mark?

Answer 1

R

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MCJ VPF YKN SA?

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English alphabet is the key:
M C J----> 13=3+10
V P F----> 22=16+6
Y K N----> 25=11+14
S A R----> 19=1+18