The hazards of oil drilling were demonstrated when an oil rig in the north sea exploded. this sentence is a) simple b) compound c) complex d) compound-complex

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.