Which parameter is indicative of how far and in what direction a reaction will progress?
a. the concentration of reactants
b. the change in free energy of the reaction (δg)
c. the concentration of products
d. the change in enthalpy of the reaction (δh)
e. the overall size of the system in which the reaction occurs

Option b, The change in free energy of the reaction (ΔG)

Explanation:

Gibbs free energy is a measure of amount of usable energy in the system.

It is related with enthalpy (H), entropy (S) and temprature (T) as:

G = H - TS

The Gibbs free energy change (ΔG) provide spontaneity of a chemical reaction.

If ΔG is negative, then reaction is spontaneous that means reaction is moving towards forward direction.

If ΔG is positive, then reaction is non-spontaneous that means reaction is moving in backward direction.

If ΔG is zero, then reaction is at equilibrium.

Change in enthalpy only gives informtion about heat involed in a chemical reaction, it does not give information about direction of the reaction.

So, among the given options, option b is correct.