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

Solution : Given:
1. Doctor  2. Doctorate   3. Docile   4. Documentary   5. Dock   6. Doctrine

Step 1: The first, second, and third letters of each word are the same, so move on to the next letter.
Step 2: The fourth letter of each word is: t, t, i, u, k, t . Based on the alphabetical order of these letters, we can arrange them: Docile, Dock, Doctor, Doctorate, Doctrine, and Documentary.
Step 3: Compare the fifth letter of (Doct o r, Doct o rate, Doct r ine). (Doctor, Doctorate) will come before Doctrine in the sequence as o comes before r in the alphabetical system.
Step 4: Compare the sixth letter of (Docto r , Docto r ate). The sixth letter of these words is the same. Doctor will come before Doctorate in the sequence as the doctor is a six–letter word and doctorate is a nine–letter word.

So, the sequence is Docile, Dock, Doctor, Doctorate, Doctrine, Documentary, or 351264. Hence, the first option is correct.