Option B is correct, With increase in concentration the density of reactants increases and the system becomes more crowded, the greater the reactants will come in contact with each other and collisions occur. If collision is in proper orientation and has optimum energy then its fruitful and yields product. So, the greater the number of reactants, the greater will be the chances of collision and the greater will be the production of products per unit time and hence, greater is the rate of reaction.
