Well golly, I’ve never thought about it til now. I always assumed it was pure butter with some short-chain molecules.
It transpires that (b) is what we have. Anchor Semi-soft butter comes in squarish tubs - 1/2 or 1 pound sizes, and contain a 30% mix of canola oil. I like it. My wife however insists on olive oil spreads (sigh).
“The person who invented spreadable butter is a New Zealander called David Illingworth who is originally from Lytham St Annes, Lancashire. I know this because he is my great uncle. He was recently featured on the clever Kiwis selection of postage stamps along with his colleague.”
I try to keep some butter at room temperature, but sometimes it gets used up without me noticing.
In my experience, this works better: put a stick of butter in the microwave along with two cups of water in a separate container. Microwave for 30-60 seconds, in 10-second intervals after the first 30 seconds.
The water moderates the microwaves, and leaves you with more evenly melted butter. Same idea, more forgiving process.
You get your butter knife, you get the amount of butter you want and then either in the butter container or on the butter lid you drag the butter back and forth until it softens.
We kept the butter out when I was a kid and we still do. I’m in my sixties, and it hasn’t killed me yet. Let him use refrigerator butter and you can keep some out for yourself. He can get his hard butter and you can get your soft butter. It’s unsalted butter, too.