In a code language, STRONG is written as TSORGN. How will REGARD be written in that language?

The logic here is the swapping of adjacent pairs of letters:
S T→T S

R O→O R

N G→G N
So, STRONG becomes TSORGN.

Applying the same logic to REGARD:

R E→E R

G A→A G

R D→D R

Thus, REGARD becomes ERAGDR.