If MISSION is coded as NJTTJPO, how is NJTTJPO coded?

Correct — A.
Rule: every letter moves one place forward (+1).

MISSION → NJTTJPO: M→N, I→J, S→T, S→T, I→J, O→P, N→O.

Apply the same +1 to NJTTJPO:
N→O, J→K, T→U, T→U, J→K, P→Q, O→P
= OKUUKQP → option (a).

Why the others are wrong

• (b)
  NKTUKPP — NKTUKPP leaves three letters unshifted: the first N, the third T and the sixth P. Under +1 they must become O, U and Q.
• (c)
  OKTUKQP — OKTUKQP leaves the third letter as T. Both T's in NJTTJPO must move to U, so the code reads UU, not TU.
• (d)
  OKUULQP — OKUULQP moves the fifth letter two places, J→L. Every letter moves exactly one, so that J becomes K.

In a letter-shift code, every letter moves the same number of places along the alphabet. Find the shift from one pair of letters, then check it on the rest before you trust it.

This question feeds the code its own output: NJTTJPO is already MISSION + 1, so the answer OKUUKQP is MISSION + 2. Doubled letters help too: TT must stay a matching pair, which rules out NKTUKPP and OKTUKQP at a glance.

Key facts

• MISSION + 1 = NJTTJPO.
• NJTTJPO + 1 = OKUUKQP, which is MISSION + 2.
• A uniform shift keeps repeated letters repeated: TT becomes UU.

Common traps

• Shifting some letters and copying the rest across unchanged, as NKTUKPP does
• Checking only the first and last letters of a seven-letter code, when OKUULQP differs in the middle

No directly related past PYQ was found.