Correct Answer: 6, 2, 5,1, 3, 4

Solution : Given:
1. Monocrystal 2. Monitoral 3. Monocular 4. Monsieur 5. Monochromist 6. Mongoose

Step 1: Compare the first, second, and third letters of each word. Since all the words start with the same letter M and are followed by o, n then move on to the next letter.
Step 2: The fourth letter of each word is g , i , o, or s . Based on the alphabetical order of these letters, we can arrange them – Mon g oose, Mon i toral, Mon o crystal, Mon o cular, Mon o chromist, Mon s ieur
Step 3: Compare the sixth letter of (Monoc r ystal, Monoc u lar, Monoc h romist). Monoc h romist will come before Monoc r ystal and Monoc u lar come in the last in the sequence as h comes before r in the alphabetical system and u comes at last.

So, the sequence is Mongoose, Monitoral, Monochromist, Monocrystal, Monocular, Monsieur or 6,2,5,1,3,4. Hence, the fourth option is correct.