The sentence "He is weak: nevertheless, he works hard" is a Compound sentence.

Here's the reason why the others are not correct:

1. Simple Sentence: A simple sentence contains a single independent clause and expresses a complete thought. The given sentence consists of two independent clauses joined by the coordinating conjunction "nevertheless," making it compound, not simple.
2. Complex Sentence: A complex sentence contains one independent clause and at least one dependent clause. The given sentence does not contain any dependent clauses, so it is not a complex sentence.
3. Negative Sentence: A negative sentence typically includes a negation word like "not." The given sentence doesn't include any negation words, so it is not a negative sentence.

The sentence "He is weak: nevertheless he works hard" is a Compound sentence. It consists of two independent clauses separated by a colon, and "nevertheless" is an adverb used to show the contrast between the two clauses.