The correct answer is A. I just watched The Lord of the Rings —my favorite film trilogy—with my friends
Step-by-step explanation:
One of the main uses of the em dash (—) is to enclose additional information, comments or remarks because this punctuation mark acts similarly to commas or parentheses that also enclose extra information. This implies, in the sentence presented, it is correct to use em dashes to enclose "my favorite film trilogy" because this information is just a remark or explanation of the importance of The Lord of the Rings, and therefore it is extra information. According to this, the first option or option A is the one that shows the correct use of the em dash.