If MASTER is coded as NBTUFS, how is NBTUFS coded?

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

MASTER → NBTUFS: M→N, A→B, S→T, T→U, E→F, R→S

Apply +1 to NBTUFS:
N→O, B→C, T→U
U→V, F→G, S→T
Code: OCUVGT → option (a).

Why the others are wrong

• (b)
  OCVTGT — The middle reads VT, but T + 1 = U and U + 1 = V, so the third and fourth letters must be UV.
• (c)
  OBTVGT — B and T are left unshifted. The rule moves every letter, so B becomes C and T becomes U.
• (d)
  OBTVFS — Only N and U are shifted. B, T, F and S stay as printed, while the rule moves every letter one place.

A uniform shift code adds the same number to every letter's position. Find the step from one pair (M 13 → N 14 is +1), confirm it on another (R 18 → S 19), then apply it.

Coding the code applies that step a second time. MASTER → NBTUFS → OCUVGT is +2 in total from the original word.

Key facts

• M(13) → N(14): the code adds 1 to each letter's position.
• NBTUFS moved +1 letter by letter is OCUVGT.
• Two +1 codings in a row equal one +2 coding: MASTER → OCUVGT.

Common traps

• Decoding (−1) instead of coding (+1), which returns MASTER
• Checking the first letter and assuming the rest

No directly related past PYQ was found.