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

Solution : Given:
1. Decline 2. Decouple 3. Decrease 4. Decorate 5. Decorum 6. Decode

Step 1: The first, second, and third letters of all the words are the same, move on to the next letter.
Step 2: Compare the fourth letter of (Dec l ine, Dec o uple, Dec r ease, Dec o rate, Dec o rum, Dec o de). Based on the alphabetical order of these letters, we can arrange them – Dec l ine, Dec o uple, Dec o rate, Dec o rum, Dec o de, Dec r ease
Step 3: Compare the fifth letter of (Deco u ple, Deco r ate, Deco r um, Deco d e). Therefore, the order is Decode, (Decorate, Decorum), Decouple.
Step 4: Compare the sixth letter of (Decor a te, Decor u m). Decorate will come before Decorum as a comes before u in the alphabetical system.

So, the sequence is Decline, Decode, Decorate, Decorum, Decouple, Decrease, or 164523. Hence, the first option is correct.