✅ Option C is a complex sentence because it contains an independent clause ("John stayed up late to finish his project") and a dependent clause ("Even though he was tired") introduced by the subordinating conjunction "even though." The dependent clause cannot stand alone as a complete thought, making it reliant on the independent clause.

❌ Option A is a simple sentence as it contains only one independent clause ("The chef prepared a delicious meal") and no dependent clauses.

❌ Option B is a compound sentence, consisting of two independent clauses ("She sang beautifully" and "the audience applauded enthusiastically") joined by a coordinating conjunction ("and").

❌ Option D is also a compound sentence, as it comprises two independent clauses ("The sun shone brightly" and "a cold wind was blowing") joined by a semicolon and a conjunctive adverb ("however").

✅ Option A is a complex sentence because it contains one independent clause ("Maria felt nervous before the final exam") and one dependent clause ("Although she studied diligently for weeks") introduced by the subordinating conjunction "Although".

❌ Option B is a compound sentence, as it contains two independent clauses ("The strong wind howled through the trees" and "the old house creaked ominously") joined by the coordinating conjunction "and".

❌ Option C is a simple sentence, containing only one independent clause with a compound predicate ("John quickly finished his chores and then watched television").

❌ Option D is a compound-complex sentence because it has at least two independent clauses ("we couldn't borrow the book" and "we found it online") and at least one dependent clause ("Since the library closed early").

✅ A complex sentence is characterized by having one independent clause and at least one dependent (subordinate) clause.

The dependent clause relies on the independent clause to make complete sense and is typically introduced by a subordinating conjunction (e.g., 'because', 'although', 'since') or a relative pronoun (e.g., 'who', 'which', 'that').

❌ Option A describes a compound sentence, which consists of two or more independent clauses joined by a coordinating conjunction (FANBOYS).

❌ Option C describes a simple sentence, which contains only one independent clause.

❌ Option D describes a comma splice, a grammatical error where two independent clauses are incorrectly joined by just a comma.

✅ Sentence C is a complex sentence because it contains one independent clause ("he still felt nervous about the exam") and one dependent clause ("Although he had studied diligently") joined by the subordinating conjunction "Although".

❌ Sentence A is a compound sentence, consisting of two independent clauses joined by a coordinating conjunction ("and").

❌ Sentence B is a simple sentence, as it contains only one independent clause and no dependent clauses.

❌ Sentence D is a compound sentence, composed of two independent clauses joined by a semicolon.