Answer:
You would use was.
Step-by-step explanation:
Using "were" in this sense, while it is past tense, doesn't make sense. It is not grammatically correct. If you notice "lunch" is not plural, meaning "were" would not fit it. You would use "was" for it to be grammatically correct.
Here's what the sentence would look like if you used were: "Lunches for the guests were prepared yesterday."
If you were to use "was", the sentence would be: "Lunch for the guests was prepared yesterday.", because the "lunch" is singular. Making this the correct sentence.
I hope this helps!
(Sorry for the mess up!)
-No one