Answer:
"How long does this film last?"
Step-by-step explanation:
Auxiliary verbs are those words that add to the meaning of a clause functionally or grammatically. They are also known as helping verbs and helps to identify the mood, voice, or tense form of the verb in the sentence.
In the given sentence, the word to be added is either "do" or "does". And the correct auxiliary verb to be used is "does".
The new sentence will become
"How long does this film last?"