In a certain code language, ‘RUG’ is coded as ‘EPS’ and ‘SIT’ is coded as ‘RQG’. How will ‘COP’ be coded in the same language?

Correct — D. Two steps, in this order.

Rule: each letter moves back 2 places, then the last letter of the result is brought to the front.

RUG: R−2 = P, U−2 = S, G−2 = E → PSE → move E to the front → EPS
SIT: S−2 = Q, I−2 = G, T−2 = R → QGR → move R to the front → RQG

COP: C−2 = A, O−2 = M, P−2 = N → AMN
Move N to the front → NAM → option (d).

Why the others are wrong

• (a)
  MBM — MBM opens with M, which is three places back from P. The shift is exactly two, so the code has to begin with N.
• (b)
  MBO — MBO ends in O, the middle letter left unshifted. Every letter drops two before the rotation, so O has to become M.
• (c)
  NBM — First and last letters are right. The middle B is one place back from C, and one place is not the shift — two places gives A, so the code reads NAM.

A three-letter code that is not a plain shift usually hides two operations: a shift applied to each letter and a rearrangement of the three. Test them one at a time.

Line the word up against the code letter for letter first. RUG against EPS gives R to E, U to P, G to S, which is no single shift — so the letters have moved position as well.

Now try the shift against the code read in a different order. R to P, U to S, G to E is a clean step of two backwards, and the code is that result with its last letter carried to the front.

Key facts

• RUG shifted back two letters is PSE, and EPS is PSE with the last letter moved to the front.
• SIT shifted back two letters is QGR, and RQG is QGR with the last letter moved to the front.
• The same two steps carry COP to AMN and then to NAM.

Common traps

• Pairing the letters by position and reading R to E as a shift of +13, which fits nothing else.
• Applying the shift and stopping there, which gives AMN and is not offered.
• Rotating the wrong way, which gives MNA rather than NAM.

No directly related past PYQ was found.