The correct punctuation mark to use in the sentence "I saw a movie last night _____ it was a romantic comedy and really enjoyed it" is **a semicolon** (;). A semicolon is used to join two independent clauses that are closely related in meaning. In this case, the two independent clauses are "I saw a movie last night" and "it was a romantic comedy and really enjoyed it." These two clauses are closely related because they both describe the same event, so a semicolon is the correct punctuation mark to use.
The other punctuation marks you mentioned, a colon (:) and a dash (-), would not be appropriate in this sentence. A colon is used to introduce a list or explanation, and a dash is used to add emphasis or to introduce a parenthetical thought. In this sentence, neither of these punctuation marks is necessary or appropriate.
Here is the sentence with the correct punctuation mark:
```
I saw a movie last night; it was a romantic comedy and I really enjoyed it.
```