* Valence Electrons and Reactivity: Alkali metals like potassium have one valence electron (electron in the outermost shell). This single electron is easily lost, making them highly reactive. They readily form positive ions to achieve a stable electron configuration.

Let's look at the other options:

* a. its conducts heat: While alkali metals are good conductors of heat (and electricity), this doesn't directly explain their high reactivity.

* b. soft and shiny: Alkali metals are soft and shiny, but these properties are related to their metallic bonding, not their reactivity.

* c. electricity: Similar to heat conductivity, being a good conductor of electricity is a characteristic of metals but doesn't explain their high reactivity.

In summary: The presence of a single valence electron is the primary reason why potassium is highly reactive.