The oxidation number of as in na3aso2 is

Step 1:
           Write down Chemical formula, Overall compound is neutral, so overall charge on it is zero,

                                              Na₃AsO₂  =  0

Step 2:
           Write down the oxidation state of known elements, i.e O.S of Na is +1, and O.S of O is -2, Now put these values in above formula.

                                        (+1)₃ + As + (-2)₂  =  0

Step 3:
           Now solve for As,

                                        (+1)₃ + As + (-2)₂  =  0

                                        +3 + As - 4  =  0

                                         As  =  4 - 3

                                         As  = +1
Result:
            Oxidation state of Arsenic in Na₃AsO₂ is +1.

Answer:

a. energy is absorbed in the reaction.

explanation:

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.