Correct Answer: NATURAL

Solution : Given:
PRONUNCIATION

First option: RATION; All the letters of RATION are present in the word PRONUNCIATION.
Second option: PRONOUN; All the letters of PRONOUN are present in the word PRONUNCIATION.
Third option: PRINT; All the letters of PRINT are present in the word PRONUNCIATION.
Fourth option: NATURAL; L is not present in the given word PRONUNCIATION. All the letters of NATURAL are not present in the word PRONUNCIATION.

So, from the above, only NATURAL cannot be formed using the letters of the word PRONUNCIATION. Hence, the fourth option is correct.