Correct Answer: DECENT

Solution : Given:
EFFLORESCENT

Now, let's compare all the option words with the word EFFLORESCENT –
First option: ORESTC; All the letters of ORESTC are present in the word EFFLORESCENT.
Second option: FOREST; All the letters of FOREST are present in the word EFFLORESCENT.
Third option: DECENT; D is not present in the word EFFLORESCENT. Therefore, all the letters of DECENT are not present in the word EFFLORESCENT.
Fourth option: SERTEO; All the letters of SERTEO are present in the word EFFLORESCENT.

So, from the above, only DECENT cannot be formed from the word EFFLORESCENT. Hence, the third option is correct.