It was ____ boring film that I fell asleep. 

Out of the options, the best way to complete the sentence "It was ____ boring film that I fell asleep" is:

It was such a boring film that I fell asleep.

Here's why:

• "As" introduces a reason or cause, but it wouldn't fit naturally in this context. You wouldn't say "It was as boring film that I fell asleep" because the degree of boredom isn't the reason you fell asleep, but rather the consequence.

• "So" can intensify an adjective, but it's often used for emphasis rather than degree. While you could say "It was so boring," it doesn't convey the strong emphasis on the specific film itself causing your sleepiness.

• "Such" adds emphasis to the following adjective. In this case, "such a" emphasizes the extreme degree of boredom the film possessed.

• "Such a" combined with the adjective intensifies the quality and makes the sentence flow more naturally when describing the cause of falling asleep.

Therefore, "such a" effectively combines emphasis with specifying the exact film that led to your slumber, making it the most appropriate choice to complete the sentence grammatically