The sincerity of pronouncements by multinationals is/are anybody's guess.
My understanding is "sincerity" is an uncountable noun, and should be followed by "is", but it does feel a little weird to use the singular form when it is modified by "multinationals" which is clearly plural.
I feel like this is a very basic grammar question and I am sure I have studied about it in my high school English class. But I don't know how to search for it since I don't know what specific grammar rules I am looking for. Thanks for the help in advance.