Correct Answer: ORANGE

Solution : Given:
ORGANISATION

Now, let's compare all the option words with the word ORGANISATION –
First option: GRANT; All the letters of GRANT are present in the word ORGANISATION.
Second option: NATION; All the letters of NATION are present in the word ORGANISATION.
Third option: GIANTS; All the letters of GIANTS are present in the word ORGANISATION.
Fourth option: ORANGE; All the letters of ORANGE are not present in the word ORGANISATION. E is not present in the word ORGANISATION.

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