In a certain code language, "NATIONAL" is written as "ONALNATI". How is "ZENITH" written in that code language?

Let's analyze the structural shift in the word "NATIONAL":
Total letters = 8.
Split the word into two equal halves of 4 letters each:
First half: NATI
Second half: ONAL

In the code "ONALNATI", the second half is placed first, and the first half is placed second:

Code=Second Half+First Half=ONAL+NATI
Now apply this rule to the word "ZENITH":
Total letters = 6.
Split into two equal halves:
First half: ZEN
Second half: ITH

Interchanging their blocks:

Code=ITH+ZEN=ITHZEN