Answer: The option that fixes this fused sentence error is the last one: I have to complete my assignment, so I can't go to the movies tonight.
Explanation: The sentence "I have to complete my assignment I can’t go to the movies tonight" contains a fused sentence error because there is no punctuation mark between the two independent clauses that form it. In that way, it is necessary to add an appropriate conjunction and a punctuation mark between both clauses in order to fix this mistake. The last option ("I have to complete my assignment, so I can't go to the movies tonight"), which includes a comma and the conjunction "so", corrects this sentence because it establishes a logical relationship between the two clauses. In this revised sentence, the first clause ("I have to complete my assignment") represents a cause, while the second sentence ("so I can't go to the movies tonight") constitutes an effect or consequence.