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

Solution : Given:
1. Irreparable 2. Irreplaceable 3. Irrigated 4. Irritable 5. Irradiation

Step 1: Compare the first, second and third letters of each word. Since all the words start with the same letter I and are followed by r , then move on to the next letter.
Step 2: The fourth letter of each word is either a, e or i. Based on the alphabetical order of these letters, we can arrange them – Irr a diation, Irr e parable, Irr e placeable, Irr i gated, Irr i table.
Step 3: Compare the sixth letter as the fifth letter is the same (Irrep a rable, Irrep l aceable) and (Irri g ated, Irri t able). Irrep a rable will come before Irrep l aceable in the sequence as a comes before l in the alphabetical system. Irri g ated will come before Irri t able in the sequence as g comes before t in the alphabetical system.

So, the sequence is Irradiation, Irreparable, Irreplaceable, Irrigated, Irritable, or 5, 1, 2, 3, 4. Hence, the first option is correct.