If MARKER is coded as NBSLFS, how is NBSLFS coded?

Correct — A.
Rule: each letter moves +1 (M → N, A → B, R → S, K → L, E → F, R → S).

The question applies the same code to the code word itself:
N → O, B → C, S → T
L → M, F → G, S → T

NBSLFS → OCTMGT → option (a).

Why the others are wrong

• (b)
  OCSMGT — OCSMGT leaves the third letter as S. Every letter moves +1, so S becomes T: OCTMGT.
• (c)
  OCTMGS — OCTMGS leaves the last letter as S. It moves +1 like the rest, to T.
• (d)
  OCTNGT — OCTNGT moves L two places to N. The rule moves every letter one place, so L becomes M.

A uniform shift code moves every letter the same number of places. MARKER → NBSLFS shows +1 at all six positions, which is worth checking letter by letter rather than assuming from the first two.

Coding the output again is simply a second shift: by the time it reads OCTMGT, MARKER has moved +2 in total.

Key facts

• MARKER → NBSLFS moves each letter one place forward.
• Coding the code again moves each letter one more place: NBSLFS → OCTMGT.
• OCTMGT is MARKER shifted +2 in total.

Common traps

• Leaving a repeated letter unshifted: NBSLFS has S twice, and both become T.
• Moving one letter two places by mistake, as option (d) does with L → N.

No directly related past PYQ was found.