The correct answer is option 1: 2, 1, 4, 3.

In this question, we are given a list of rivers (List I) and a list of corresponding mouths of the rivers (List II). We need to match the correct river with its mouth. Let`s analyze each option to understand why option 1 is correct.

Option 1: 2, 1, 4, 3

In this option:

- River Danube (A) - Matches with mouth 2 (Black Sea) - This is correct as the Danube flows into the Black Sea.

- River Rhine (B) - Matches with mouth 1 (North Sea) - This is correct as the Rhine flows into the North Sea.

- River Rhone (C) - Matches with mouth 4 (Mediterranean Sea) - This is correct as the Rhone flows into the Mediterranean Sea.

- River Loire (D) - Matches with mouth 3 (Bay of Biscay) - This is correct as the Loire flows into the Bay of Biscay.

Therefore, option 1 correctly matches each river with its respective mouth.

Alert - correct answer should be option 1.