Which of the following sentences is a complex sentence?
a. i ate dinner but not lunch.
b. i ate dinner.
c. although i like to go out for dinner, i have not had time, and i don’t have anyone to take with me.
d. who wants to eat lunch before noon?

A). I ate dinner but not lunch.

Explanation:

Complex sentences are elucidated as the sentences that consists of a dependent or subordinate clause joined with an independent clause. These sentences convey a complex idea as it comprises of combining a stronger idea(through independent clause) with a weaker idea(dependent or subordinate clause).

In the given question, the first sentence i.e. 'I ate dinner but not lunch' exemplifies a complex sentence as it consists of an independent clause 'I ate dinner' followed by the dependent clause 'but not lunch' which is dependent on main clause to accomplish its meaning. These clauses are combined together to convey a complex thought. Thus, option A is the correct answer as the other options are either simple sentences(option B), interrogative(option D) or exemplifies compound-complex sentence(option A involves a dependent and two independent clauses).