2. Scientists use seismometers to measure the earthquake activity that occurs beneath a volcano. They then predict the eruption of that volcano.
Explanation:
The options are:
1. Before a volcano erupts, earthquake activity beneath the volcano decreases.2. Scientists use seismometers to measure the earthquake activity that occurs beneath a volcano. They then predict the eruption of that volcano.3. When a volcano erupts, the amount of carbon dioxide and sulfur dioxide emitted decreases. 4.Scientists measure the amount of these gases to determine the amount of magma present in the volcanic reservoir.
The answer is certainly 2. as seismometers are being used to find out the details of the earthquake activities which occur inside a volcano. And with this information, the volcanologists then predict the eruption of that particular volcano. Hence, the 2nd option does not have any factual errors, and all the others have factual errors.