The correct option is D.
"My computer was working very slowly; I had to restart it for the updates to take effect" is the correct option because we have two independent clauses combined by means a semicolon.
- Option A is incorrect because it uses a comma to join independent clauses.
- Option B is incorrect because there is no comma preceding the conjunction "and".
- Opcion C is incorrect because there is no punctuation mark, or a conjunction to combine the sentence.