Is a mollusk closest to a reptile or a mammal or amphibian or fish?

The mollusk phyllum (which includes snails, slugs, octupus, squid and some other animals) are more like related to fish.

Both groups live in or have to stay close to water (notice how snails/slugs dry out if they get in the sun too long). Mollusk are all invertebrate animals (no backbone), while today's fish are indeed vertebrates. Some of the jawless fish (like the Hagfish or the Lamrey Eel) have a very flexible vertebrate that would almost appear as if they didn't have one if you were to grab them.