Looking into the alphabets there are six such pairs namely ON, HONE, ST, TRAPHO, TRAPHON, RAP.

1. ON  – NO

2. HONE – EFGH

3. ST-ST

4. TRAPHO – OPQRST

5. TRAPHON – NOPQRST

6. RAP – QPR

As per the question, the interchanges take place as follows:

Here we can see that Q interchanges with S. Then to the left of Q, the 5th letter would be P because of P interchanges with N.

Sixth left of 15^th from left

15 – 6 = 9

Or 9^th  from left

The given series is a mixed series. The pattern for the first letters:

The given sequence consists of two series :
I.    Z, W, T, V, Q, ? in which each letter is moved three steps backward to obtain the next term.
II.   S, O, K, G in which each letter is moved four steps backward to obtain the next term.

The first letters are in alphabetical order with a letter skipped in between each segment: C, E, G, I, K. The second and third letters are repeated; they are also in order with a skipped letter: M, O, Q, S, U.

There are two alphabetical series here. The first series is with the first letters only: STUVW. The second series involves the remaining letters: CD, EF, GH, IJ, KL.

The letter of the word are written in a reverse order and each letter, except the first and the last one, is moved one step forward, to obtain the code

S      U       G       A        R
R     P         K       L         U

From the above, we get

S = R

U = P

G = K

A = L

R = U

Similarly,

G       U     R        U       S
K        P     U        P       R

Z+3=C-2=A+3=D, X+3=A-2=Y+3=B, V+3=Y-2=W+3=Z,_______ T + 3 = W – 2 = U + 3 = X => TWUX