The correct anwer is "who" and the sentence would complete as follows:
"There was an actress on television last night who is starring in a new film."
In this case, the pronoun who is making reference to "the actress".
The pronoun "who" should be used as the subject of a sentence as in the example "who is starring a new film" where "who" is the subject, "is starring" is the verb, and the rest is the predicate. Also, the word "who" can be replaced by a "she" so we can tell that whe pronoun "who" is the correct one.