Which letter trio does NOT follow the ascending order? ABC, BCD, CDE, DFE, EFG

Correct — A.
Rule: each trio is three consecutive letters in alphabetical order.

ABC, BCD, CDE and EFG each step forward one letter at a time.
In DFE the positions run 4, 6, 5: F comes before E, so the order falls at the end.

The trio should read DEF → option (a).

Why the others are wrong

• (b)
  CDE — CDE runs 3, 4, 5 in alphabet positions, each one more than the last. It is in ascending order.
• (c)
  BCD — BCD runs 2, 3, 4, a clean forward step each time. Nothing in it is out of order.
• (d)
  ABC — ABC is the first three letters of the alphabet in order, 1, 2, 3. It sets the pattern the other trios follow.

Turn letters into positions (A = 1, B = 2 …) and ascending order becomes a question about numbers: each position must be larger than the one before.

The list has a second pattern too: each trio starts one letter later than the last, at A, B, C, D, E. So the fourth trio should be D, E, F. DFE holds the right letters in the wrong order, with E and F swapped.

Key facts

• Positions: D = 4, E = 5, F = 6.
• DFE reads 4, 6, 5, falling from 6 to 5.
• The trios start at A, B, C, D, E in turn, so the fourth should be DEF.

Common traps

• Checking that each trio has the right letters without checking their order

No directly related past PYQ was found.