Explanation:

- The pattern of the code can be discerned by looking at the examples given.

- For 'HALMST' to 'IZMLTS', each letter shifts by a specific value:

- H to I (+1), A to Z (-1), L to M (+1), M to L (-1), S to T (+1), T to S (-1).

- In 'GEASTR' to 'HDBRUQ', the shift is similar:

- G to H (+1), E to D (-1), A to B (+1), S to R (-1), T to U (+1), R to Q (-1).

- From the above, it is clear there is an alternating pattern of +1, -1.

- For 'TRAILS':

- T becomes U (+1)

- R becomes Q (-1)

- A becomes B (+1)

- I becomes H (-1)

- L becomes M (+1)

- S becomes R (-1)

UQBHMR

- The correct answer is option 4: UQBHMR.