The correct answer is this: THE PHYSICAL STATE OF EACH REACTANT AND PRODUCTS.
When writing chemical equations, chemists usually use lower case alphabets to indicate the physical state of each of the reactants and products. This is done in order to provide information about the conditions of the reaction. Typically, lower case 's' is used to represent solid, 'l' is used to represent liquid while 'g' is used to represent gas. Indicating the states of matter of chemical compounds make it easy for one to tell whether a reaction is an exothermic or an endothermic reaction.
