Correct Answer: PIRATE

Solution : Given:
SEPARATED

Now, let's compare all the option words with the word SEPARATED –
First option: RATE; All the letters of RATE are present in the word SEPARATED.
Second option: REPEAT; All the letters of REPEAT are present in the word SEPARATED.
Third option: SEAT; All the letters of SEAT are present in the word SEPARATED.
Fourth option: PIRATE; I is not present in the word SEPARATED. Therefore, all the letters of PIRATE are not present in the word SEPERATED.

So, PIRATE cannot be formed with the letters of the word SEPARATED. Hence, the fourth option is correct.