Answer:
(b) 1 equivalent
Explanation:
Concentration is the relative amount of a substance in a given volume or mass of something else.
An equivalent is not a concentration, because it is just a mass —the mass of a substance needed to react with or supply one gram of hydrogen.
(a) is wrong. 1 M means that there is 1 mol of a substance in 1 L of solution.
(c) is wrong. 1 % by mass that there is 1 g of a substance in 100 g of the mixture.
(d) is wrong. 1 m means that there is 1 mol of a substance in 1 kg of solvent.