If USA is DSA and SOS is ERS, what is ISD ?

Question

Explain the loc please.

Answer 1

FSD or FZD or FXD?

USA -> DSA
SOS-> ERS

The 3rd letter doesn't change, so from ISD, D->D

The 1st letter changes in alphabetical order from the first to the second word, so in the series D,E, the next letter is F.

The second letter remains the same in the first "word" and changes by a 2 letters gap in the second O-PQ-R.
However we can't really define the pattern; it could be alternating 0 change, 2-letter gap change, so for the 3rd "word" you have S remaining S (then you have FSD), or you could have an increasing gap e.g. the gap could be (2^n)-2 letters.
Then for the first word n=1 =>2^1-2 = 2-2=0 => no change
for the second n=2 => 2^2-2= 4-2 =2 letters
thus for the third n=3 => 2^3-2= 8-2 = 6 letters so S-TUVWXY-Z and you get FZD.
or you could have 2n-2 letter gap
Then for n=1, 2*1-2 =2-2 =0 => no change
for the second n=2, 2*2-2= 4-2= 2
thus for n=3, 2*3-2= 6-2= 4, thus S-TUVW-X and you get FXD