Complex Sentence
In this sentence, the subject "he" is performing the action of putting hard work, but the main clause of the sentence states that he could not complete the task. This sentence is considered complex because it consists of an independent clause ("he could not complete the task") and a dependent clause ("In spite of putting hard work").

Explanation:
- Complex Sentence: A complex sentence is a sentence that contains one independent clause and at least one dependent clause. The independent clause can stand alone as a complete sentence, but the dependent clause cannot.
- Subject and Action: The subject "he" is performing the action of putting hard work.
- Main Clause: The main clause of the sentence is "he could not complete the task," which is an independent clause.
- Dependent Clause: The dependent clause in this sentence is "In spite of putting hard work," which adds context to the main clause but cannot stand alone as a complete sentence.
- Result: The overall meaning of the sentence is that despite the hard work put in by the subject, he was unable to complete the task.
Overall, this sentence is complex as it combines both an independent and dependent clause to convey a complete thought.