If "CANADA" is written as "ADANAC", what is "MEXICO" written as?

Correct — B.
Rule: write the word backwards: the last letter comes first, the first goes last.

CANADA read from the right: A-D-A-N-A-C = ADANAC
MEXICO read from the right: O-C-I-X-E-M

That reads OCIXEM → option (b).

Why the others are wrong

• (a)
  OCIXME — OCIXME ends M, E, copying the first two letters of MEXICO forwards. Reversal sends the first letter to the end, so the code must close on E, M.
• (c)
  OCXEIM — OCXEIM puts X third. Read backwards, MEXICO gives O, C, then I (its fourth letter) before X (its third).
• (d)
  OCMXIE — OCMXIE moves M into third place. M is the first letter of MEXICO, so reversal sends it to the end of the code.

A reversal code keeps every letter and changes only the order: letter 1 goes last, letter 2 second-last, and so on.

Test it on the given pair first. CANADA has six letters, and read from the right it is ADANAC. MEXICO also has six, so the same positions 6-5-4-3-2-1 apply: O, C, I, X, E, M.

Key facts

• Reversal of a six-letter word takes positions 6-5-4-3-2-1.
• A pure rearrangement keeps every letter, so all four options here are anagrams of MEXICO.
• Check the ends first: the code must start with the word's last letter (O) and end with its first (M).

Common traps

• Reversing MEXICO but copying its first two letters forwards, which gives OCIXME
• Checking only the opening O and C, which all four options share

No directly related past PYQ was found.