An element is the smallest part of a substance that is capable of independent existence. An element cannot be broken down into any other substance. Krypton, Bismuth etc are all elements.
A compound is a combination of two or more elements which are chemically combined together e.g NH3, CO2 etc.
A mixture is any combination of substances that are not chemically combined together. E.g Pizza, milk etc
The answer is (b). As, vanadium is attached to five fluoride atoms, each flouride containing -1 oxidation state, hence five fluoride contains -5, to neutralize, vanadium should have +5 oxidation state.