Correct option is B
Observe pattern:
UNDERSTAND
ERST → middle part
NDERSTAN → word without first and last letter
RS → middle of previous small part
Pattern is alternating:
Big word → middle → big word (trimmed) → middle → again trimmed version
From:
NDERSTAN
If we remove first and last letter again:
NDERSTAN
Remove N and N → DERSTA
So next term = DERSTA
Correct Answer: (B) DERSTA