Which one of the following statements ia an example of a complex sentence?

a. Neap time is the best time for digging clams.

b. Sparrows at the bird feeder compete for seeds.

c. The Earth was spinning when Lucas first saw Lucille.

d. The crescent moon is a symbol of Islam.

I believe that (C) The Earth was spinning when Lucas first saw Lucille.

A complex sentence consists of one independent clause and at least one dependent clause. I think the independent clause here is "The Earth was spinning" and the dependent clause is "when Lucas first saw Lucille". The dependent clause cannot stand on its own.