4. which statement is true about alkali metals? a. some of them explode when exposed to water. b. they are not conductors of heat or electricity. c. they are not very reactive. d. they are in group 18 of the periodic table

The correct answer is option A, that is, some of them explode when exposed to water.  

The alkali metals refer to a group in the periodic table comprising the chemical elements sodium, lithium, rubidium, potassium, francium, and cesium. This group is situated in the s-block of the periodic table. The alkali metals are all soft, shiny, highly reactive metals at standard pressure and temperature, and easily lose their outermost electron to produce cations with charge +1.  

The alkali metals are the most reactive metals found in the periodic table, they all react dynamically or some even explosively with cold water, leading to the displacement of hydrogen

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.