Explanation:

Let's decode the pattern used in the given code:

- NATURE is coded as MCQYMK.

- Notice that:

- N becomes M: N (14) - 1 = M (13)

- A becomes C: A (1) + 2 = C (3)

- T becomes Q: T (20) - 3 = Q (17)

- U becomes Y: U (21) + 4 = Y (25)

- R becomes M: R (18) - 5 = M (13)

- E becomes K: E (5) + 6 = K (11)

Using this pattern on CREST:

- C becomes B: C (3) - 1 = B (2)

- R becomes T: R (18) + 2 = T (20)

- E becomes B: E (5) - 3 = B (2)

- S becomes W: S (19) + 4 = W (23)

- T becomes O: T (20) - 5 = O (15)

So, CREST is coded as BTBWO.

- Option:1, CPHOY: Incorrect. Does not match derived code.

- Option:2, BTBOY: Incorrect. Does not match derived code.

- Option:3, BTHKO: Incorrect. Does not match derived code.

- Option:4, BTBWO: Correct. Matches derived code.

.