If SCHOOL is coded as TIDPPM, how is TIDPPM coded?

Correct — B.
Rule: each position keeps its own step. SCHOOL → TIDPPM moves the letters +1, +6, −4, +1, +1, +1.

S→T +1, C→I +6, H→D −4, O→P +1, O→P +1, L→M +1

Apply the same steps to TIDPPM:
T + 1 = U, I + 6 = O, D − 4 = Z (wrapping back past A)
P + 1 = Q, P + 1 = Q, M + 1 = N

That spells UOZQQN, option (b).

Why the others are wrong

• (a)
  UJEQQN — UJEQQN moves every letter of TIDPPM by +1. The example does not: C → I is +6 and H → D is −4, so the second and third letters must move by those steps.
• (c)
  TJEQQN — TJEQQN leaves T unmoved, though S → T in the example moves the first letter +1. It also moves I and D by +1 instead of +6 and −4.
• (d)
  UJEQPN — UJEQPN moves I and D by +1 instead of +6 and −4, and leaves the second P unmoved where the fifth letter must move +1.

A one-example letter code fixes a step for each position. Write the positions of both words, subtract column by column, and if the steps differ, keep the whole list and apply it in order to the new word.

Here four columns move +1, but C → I is +6 and H → D is −4. Counting back 4 from D passes A and lands on Z, because the alphabet wraps round.

Key facts

• SCHOOL → TIDPPM moves the letters +1, +6, −4, +1, +1, +1.
• Counting back past A wraps to Z, so D − 4 = Z.
• A uniform +1 on SCHOOL would give TDIPPM.

Common traps

• Seeing +1 in four columns and applying +1 to all six letters, which gives UJEQQN.
• Stopping at A when counting back from D instead of wrapping round to Z.

No directly related past PYQ was found.