Answer: B. It gives background for the more practical information on volcanoes that follows.
Step-by-step explanation:
There is no passage attached nor referenced as such I could not find it. The above option however, should be the answer.
When a section of a passage gives background information about what is to follow, it lays the groundwork and gives the reader context about what is being described thereby making the essay clearer. Option B is therefore the most probable answer.