In the following sentence, is the correct verb is or are?Prescriptively, that is plural for sure — the structure is "X, Y, and Z are". But this sounds pretty normal to me, the sort of thing you'd all but expect to read in a letter of this type in business.
Your , great spirit, and enthusiasm towards your internship is greatly appreciated.
But "towards your internship" seems like it applies to all three traits, and I wonder if that might make it feel more like that whole chunk is singular. Our contributor Monica points out that when the last element in a list like that is heavy, you get a kind of 'recency effect': The first two items as very short, but the third one is pretty long ("enthusiasm towards your internship"), so that people might be inclined to make the verb agree just with that part. Anyway, whatever the verb agreement, the recipient of that letter should be one happy intern.
Image from the cover of my favorite Dilbert book. Oh who am I kidding … I love them all equally.