Answer: The sentence is c)
A complex sentence
The sentence is complex because it has a dependent clause (when an oil rig in the North Sea exploded) introduced by the subordinating conjunction when and it is joined to an independen clause:
[The hazards of oil drilling were demonstrated (when an oil rig in the North Sea exploded.)]
The sentence (when an oil rig in the North Sea exploded) is thought to be a dependent clause because it does not contain a complete thougth or idea, and it is related to the independent clause, which in turn, represents a complete idea and can stand on its own. In other words, the independent clause does not require futher information because it is complete.
