It's "sup", but "sop" is acceptable as well. This is a kind of food which has clear soup with spices and other ingredients.
Meanwhile, "soto" is a dish with coconut extract added to the soup which makes it more cloudy that the previous one.
I've heard it pronounced both ways, and most Indonesians consider soup and soto to be different things even though for westerners soto is just a type of soup
Why is "sup" spelled with a "u" if it is pronounced like "up" in English? Shouldn't it be spelled with an "a" to be pronounced that way?