CaCO3 is a salt of a strong base and weak acid, which is why it is basic in nature. In order to remove it, we need to use an acidic solution. So, statement 1 is correct.
The effervescence is caused by the release of carbon dioxide gas (which forms during the reaction) and not hydrogen gas. Statement 2 is wrong.
As the reaction proceeds, the pH does not go down as we have a base and an acid reacting. So statement 3 is wrong.
The reason we are using a cleaning agent is to remove the limescale. In order to do this, we need to convert the limescale into something more soluble. We know that a salt is formed when reacting a base and an acid. Therefore, the salt has to be more soluble than limescale so it can be removed from the bathroom surfaces. So, 4 is correct.