In a certain code language, 'ABUNDANT' is written as 'ACWQGCOT', and 'RELIABLE' is written as 'RFNLDDME'. How will 'MERCIFUL' be written in that language?

Let's study the positional modifications from the original word to the code word:
For ABUNDANT→ACWQGCOT:

A→A (+0)

B→C (+1)

U→W (+2)

N→Q (+3)

D→G (+4)

A→C (+2?) Let's recheck this shift pattern:
Let's check the letters index by index:

1st:A→A(+0)

2nd:B→C(+1)

3rd:U→W(+2)

4th:N→Q(+3)

5th:D→G(+3)

6th:A→C(+2)

7th:N→O(+1)

8th:T→T(+0)

Wow, the increments follow a symmetric mountain peak pattern: +0,+1,+2,+3,+3,+2,+1,+0.

Let's verify this pattern on RELIABLE (RFNLDDME):

R+0=R

E+1=F

L+2=N

I+3=L

A+3=D

B+2=D

L+1=M

E+0=E
The pattern matches exactly.

Now, let's apply this symmetric shifting to MERCIFUL:

M+0=M

E+1=F

R+2=T

C+3=F

I+3=L

F+2=H

U+1=V

L+0=L

The resulting code is MFTFLHVL.