Most-reactive-first activity series:

The activity series of metals ranks elements based on their chemical reactivity, specifically their ability to lose electrons and form positive ions. Metals at the top of the series are highly reactive and easily oxidized, while those at the bottom are stable and 'noble'. 1. **Group 1 (Alkali Metals):** These are the most reactive. Potassium ($K$) is more reactive than Sodium ($Na$) because its valence electron is further from the nucleus. 2. **Group 2 (Alkaline Earth Metals):** These follow Group 1. Calcium ($Ca$) is more reactive than Magnesium ($Mg$). 3. **Transition Metals:** These are generally less reactive. Copper ($Cu$) is much less reactive than the main-group metals above, and Gold ($Au$) is one of the least reactive metals known. Following this trend, the correct order from most reactive to least reactive is: $$K > Na > Ca > Mg > Cu > Au$$ Therefore, option D is the correct sequence.