Correct Answer: MOMENTUM

Solution : Given:
ENVIRONMENT

Now, let's compare all the option words with the word ENVIRONMENT –
First option: ENTER; All the letters of ENTER are present in the word ENVIRONMENT.
Second option: METRE; All the letters of METRE are present in the word ENVIRONMENT.
Third option: IRON; All the letters of IRON are present in the word ENVIRONMENT.
Fourth option: MOMENTUM; All the letters of MOMENTUM are not present in the word ENVIRONMENT. U and three M are not present in the word ENVIRONMENT.

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