Which of the following words cannot be formed using the letters of the word ‘RECOMMENDATION’?

The correct option is MONSTER.

To find out which word cannot be formed, let me count the frequency of each letter in the word RECOMMENDATION:
• R: 2
• E: 2
• C: 1
• O: 2
• M: 2
• N: 2
• D: 1
• A: 1
• T: 1
• I: 1

Now let's check the letters of each given word option:
1. MEDICO: Uses M, E, D, I, C, O (all available in RECOMMENDATION).
2. DOMAIN: Uses D, O, M, A, I, N (all available in RECOMMENDATION).
3. ACTION: Uses A, C, T, I, O, N (all available in RECOMMENDATION).
4. MONSTER: Contains the letter 'S', which does not exist in the word RECOMMENDATION.

Therefore, the word MONSTER cannot be formed using the letters of the given word.