What is a verb form ending in -ing and used as a noun.?

Which statement is an example of a complex sentence? a. although he denies them, dr. vasquez faced several scandals last year. b. people tend to use too many clauses in sentences that should be simple. c. cars tend to be expensive, but airplanes are in a price range of their own. d. i am in favor of proposition 1, and i am also in favor of proposition 16.