Correct Answer: tlkkttl

Solution : Given:
_ t k _ t t _ l t t _ l t _ k l _ t k _

To fill the series we have to divide the series – _ t k _ / t t _ l / t t _ l / t _ k l / _ t k _

Let's check each option –
First option: tlkkttl; t t k l / t t k l / t t k l / t t k l / t t k l (ttkl is repeated)
Second option: tllkttl; t t k l / t t l l / t t k l / t t k l / t t k l (No repeated pattern has been found)
Third option: tlktttl; t t k l / t t k l / t t t l / t t k l / t t k l (No repeated pattern has been found)
Fourth option: tlkktll; t t k l / t t k l / t t k l / t t k l / l t k l (No repeated pattern has been found)

So, the series becomes→ttklttklttklttklttkl. Hence, the first option is correct.