is trumpet a B flat instrument? Is saxophone a "horn"? and are all strings c instruments like flute or piano? thankz

All strings are C instruments, although I suppose you could tune a string to be any sort of key. But there's no use in tuning, for example, the A string to be anything else but A. So unlike clarinets, for example, whose written music is one whole note higher than "concert" instruments, such as flute, trombone, bassoon, baritone, you could say all strings are in "concert" key. They can play piano music and it'll sound in the same key.

A Bb trumpet is Bb (common trumpets) but there are also C trumpets and stuff.

The "horn" issue is a bit of a sticky question. Some refer to all wind instruments as horns, while some insist that horn only refers to the "french horn". Saxophones can be called horns, such as, "wow, what a pretty new horn he got" or in generic things such as "horns up!" refering to many instruments, but when specifically talking about saxophones, it's better to say saxophone or sax. (e.g. "she plays the sax" or saxophones, play this")

I agree. I always talk about "my horn" altho I play a bassoon, but it's just an easier thing to say, that's all.