4a. D - The last number in the configuration represents the group, in this case 3, and the number of shells represent the period, which is also 3.
4bi. Rubidium metal reacts very rapidly with water to form a colourless solution of rubidium hydroxide (RbOH) and hydrogen gas (H2). The resulting solution is basic because of the dissolved hydroxide. The reaction is very exothermic and explosive.
4bii. Lithium is smaller in size than potassium . so valence electrons are more tightly held in Li than K. Due to larger size of K the valence electrons are loosely held and easy to remove the electron from K. Hence K is more reactive.